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A theory is developed for the optical pumping of alkali atoms with allowance for Zeeman and hyperfine structures and atomic motion in the model of instantaneous mixing between the Zeeman and hyperfine sublevels in an excited state. The appearance of the angular momentum in the 87Rb and 133Cs atoms that is opposite to the momentum of the photons pumping these atoms is studied. This effect is found to be most pronounced in the D2 line. The influence of the buffer gas pressure and the optical pumping intensity on the features of this effect is analyzed. The influence of the counterpolarization effect on the parameters of high-sensitivity quantum magnetometers is discussed. more...

The reaction kinetics of the isomers of the methylallyl radical with molecular oxygen has been studied in a flow tube reactor at the vacuum ultraviolet (VUV) beamline of the Swiss Light Source storage ring. The radicals were generated by direct photodissociation of bromides or iodides at 213 nm. Experiments were conducted at room temperature and low pressures between 1 and 3 mbar using He as the buffer gas. Oxygen was employed in excess to maintain near pseudo-first-order reaction conditions. Concentration–time profiles of the radical were monitored by photoionisation. For the oxidation of 2-methylallyl (2-MA) and with k(2-MA + O2) = (5.1 ± 1.0) × 1011 cm3 mol−1 s−1, the rate constant was found to be in the high-pressure limit already at 1 mbar. In contrast, 1-methylallyl exists in two isomers, E- and Z-1-methylallyl. We selectively detected the E-conformer as well as a mixture of both isomers and observed almost identical rate constants within the uncertainty of the experiment. A small pressure dependence is observed with the rate constant increasing from k(1-MA + O2) = (3.5 ± 0.7) × 1011 cm3 mol−1 s−1 at 1 mbar to k(1-MA + O2) = (4.6 ± 0.9) × 1011 cm3 mol−1 s−1 at 3 mbar. While for 2-methylallyl + O2 no previous experimental data are available, the rate constants for 1-methylallyl are in agreement with previous work. A comparison is drawn for the trends of the high-pressure limiting rate constants and pressure dependences observed for the O2 recombination of allylic radicals with the corresponding reactions of alkyl radicals. more...

In this work, we show that van der Waals molecules X-RG (where RG is the rare gas atom) may be created through direct three-body recombination collisions, i.e., X + RG + RG $\rightarrow$ X-RG + RG. In particular, the three-body recombination rate at temperatures relevant for buffer gas cell experiments is calculated via a classical trajectory method in hyperspherical coordinates [J. Chem. Phys. 140, 044307 (2014)]. As a result, it is found that the formation of van der Waals molecules in buffer gas cells (1 K $\lesssim T \lesssim 10$ K) is dominated by the long-range tail (distances larger than the LeRoy radius) of the X-RG interaction. For higher temperatures, the short-range region of the potential becomes more significant. Moreover, we notice that the rate of formation of van der Walls molecules is of the same order of magnitude independently of the chemical properties of X. As a consequence, almost any X-RG molecule may be created and observed in a buffer gas cell under proper conditions., Comment: 10 pages, 8 figures, submitted to the Journal of Chemical Physics more...

We present an extensive range of quantum calculations for the state-changing rotational dynamics involving two simple molecular anions which are expected to play some role in evolutionary analysis of chemical networks in the Interstellar environments, C$_2$H$^-$($X^1\Sigma^+$) and C$_2$N$^-$ ($X^3 \Sigma^-$) but for which inelastic rates are only known for C$_2$H$^-$. The same systems are also of direct interest in modelling selective photo-detachment (PD) experiments in cold ion traps where the He atoms function as the chief buffer gas at the low trap temperatures. This study employs accurate, \textit{ab initio} calculations of the interaction potential energy surfaces (PESs) for these anions, treated as Rigid Rotors (RR) and the He atom to obtain a wide range of state-changing quantum cross sections and rates at temperatures up to about 100 K. The results are analysed and compared for the two systems, to show differences and similarities between their rates of state-changing dynamics. more...

Great advances in precision quantum measurement have been achieved with trapped ions and atomic gases at the lowest possible temperatures. These successes have inspired ideas to merge the two systems. In this way one can study the unique properties of ionic impurities inside a quantum fluid or explore buffer gas cooling of the trapped ion quantum computer. Remarkably, in spite of its importance, experiments with atom-ion mixtures remained firmly confined to the classical collision regime. We report a collision energy of 1.15(0.23) times the $s$-wave energy (or 9.9(2.0)~$\mu$K) for a trapped ytterbium ion in an ultracold lithium gas. We observed a deviation from classical Langevin theory by studying the spin-exchange dynamics, indicating quantum behavior in the atom-ion collisions. Our results open up numerous opportunities, such as the exploration of atom-ion Feshbach resonances, in analogy to neutral systems., Comment: 8 pages, 6 figures including appendices more...

A number of experiments have been performed on light slowing and stopping in the electromagnetically induced transparency (EIT) media of hot atomic gases where the Doppler-shift may add detunings to the field frequencies in an inhomogeneous fashion. We provide here a theoretical analysis as to the effect of such a Doppler-broadened environment on the dynamics of the system in comparison to a cold atomic medium. We will show that one of the most critical factors in the formation of a stationary light pulse is the enhancement of the excited-state decay rate caused by the collisions between the atoms of the medium and the molecules of the buffer gas. more...

The spectral features of the velocity of light-induced drift (LID) of lithium atoms in a neon buffer gas have been theoretically studied using four known ab initio calculated interatomic interaction potentials for the Li–Ne system of colliding particles. The calculations with each of these four potentials predict a strong temperature dependence of the spectral shape of the LID signal of Li atoms in the Ne atmosphere in various temperature intervals. The results allow the high-precision test of the interatomic interaction potentials in experiments on anomalous LID. more...

8 pags., 5 figs., Is the rise of the rate constant measured in laval expansion experiments of OH with organic molecules at low temperatures due to the reaction between the reactants or due to the formation of complexes with the buffer gas? This question has importance for understanding the evolution of prebiotic molecules observed in different astrophysical objects. Among these molecules methanol is one of the most widely observed, and its reaction with OH has been studied by several groups showing a fast increase in the rate constant under 100 K. Transition state theory doesn't reproduce this behavior and here dynamical calculations are performed on a new full dimensional potential energy surface developed for this purpose. The calculated classical reactive cross sections show an increase at low collision energies due to a complex forming mechanism. However, the calculated rate constant at temperatures below 100 K remains lower than the observed one. Quantum effects are likely responsible for the measured behavior at low temperatures., We acknowledge the support of the Ministerio de Economía y Competitividad (SPAIN), under grants No. FIS2014-52172-C2 and FIS2017-83473-C2, and from the European Research Council under the European Union’s Seventh Framework Programme (FP/2007-2013)/ERC Grant Agreement no. 610256 (NANOCOSMOS). The authors thankfully acknowledges the computer resources at finisterrae2 (CESGA) and technical support provided by CESGA (AECT-2018-2-0001). Finally, the COST action CM- 1401, “Our Astro-Chemical History: Past, Present, and Future” is also acknowledged. more...

Electronic spectra are measured for mass-selected C+2( = 6–14) clusters over the visible and near-infrared spectral range through resonance enhanced photodissociation of clusters tagged with N2 molecules in a cryogenic ion trap. The carbon cluster cations are generated through laser ablation of a graphite disk and can be selected according to their collision cross section with He buffer gas and their mass prior to being trapped and spectroscopically probed. The data suggest that the C+2( = 6–14) clusters have monocyclic structures with bicyclic structures becoming more prevalent for C+22 and larger clusters. The C+2 electronic spectra are dominated by an origin transition that shifts linearly to a longer wavelength with the number of carbon atoms and associated progressions involving excitation of ring deformation vibrational modes. Bands for C+12, C+16, C+20, C+24, and C+28 are relatively broad, possibly due to rapid non-radiative decay from the excited state, whereas bands for C+14, C+18, C+22, and C+26 are narrower, consistent with slower non-radiative deactivation. more...

The dynamics of spin projections of the electron shell of an alkali metal on the coordinate axis is considered in the electron paramagnetic resonance scheme with continuous pumping by biharmonic circularly polarized laser radiation. The working region is a cell with alkali vapor metal vapors and a buffer gas at a high concentration at temperature 60°C. It was found that the use of biharmonic pumping causes not only the expected electron-spin precession, but also pulsations of the electron-spin projection on the axis along which the magnetic field is directed. The frequency of these pulsations depends on the nuclear angular momentum of alkali metal atoms. In the case of the transverse electron magnetic resonance, this effect is absent. more...

Magneto-optical resonances of electromagnetically induced absorption (EIA) in a cell filled with the vapor of alkali atoms and buffer gas are considered. The atoms are excited using a configuration of electromagnetic fields composed of two counter-propagating linearly polarized light waves and a static magnetic field. We focus on the influence of the residual magnetic field on the parameters of nonlinear resonances. In real experiments, a residual field is always present in the cell due to imperfect magnetic-field shielding. On the basis of calculations, we formulate practical recommendations for controlling the residual magnetic field in the experiments aimed at observation of high-quality EIA resonances. The results from this work can find application in quantum magnetometry and nonlinear optics. more...

A theory of excitation of a coherent population trapping resonance in hot alkali-metal atoms in a cell with buffer gas is constructed, taking into account the finite spectral width of two-component radiation, its vector properties, and the hyperfine and Zeeman structures of atoms. The propagation of intermode correlations in an optically dense gas cell is studied. It is found that in the case of partially correlated input modes, their correlation degree at the output can increase. This effect can be used for filtering the incoherent part of radiation. more...

The screening of macroparticles in a nonequilibrium plasma is considered on the basis of the Vlasov kinetic equations supplemented with the collisional term in the Bhatnagar–Gross–Krook approximation and the point sink method. In this method, the absorption of electrons and ions by a macroparticle is described by introducing effective point sinks into the kinetic equations for plasma electrons and ions. Explicit expressions are derived for the potential distribution around a macroparticle and effective charges that determine the behavior of the potential at large distances taking into account collisions of electrons and ions with neutral buffer gas atoms. We consider the cases of a constant collision frequency and constant mean free paths of electrons and ions in the buffer gas. Numerical calculations are performed for dusty isothermal and nonisothermal plasmas in helium, neon, argon, krypton, and xenon at pressures of 10–1 to 104 Pa, which are typical of experiments with dusty plasmas. more...

We studied selective reflection (SR) of laser radiation from a window of a nanocell with thickness L ~ λ1,2/2 filled with Rb and Cs atoms, where λ1 = 780 nm and λ2 = 852 nm are the wavelengths resonant with the D2 laser lines for Rb and Cs, respectively. It is demonstrated that the negative derivative of the SR signal profile for L > λ/2 changes to the positive one for L < λ/2. It is shown that the real-time formation of the SR signal profile derivative (SRD) with the spectral width 30–40 MHz and located at the atomic transition is, in particular, a convenient frequency marker of D2 transitions in Rb and Cs. The amplitudes of SRD signals are proportional to the atomic transition probabilities. A comparison with the known saturated absorption (SA) method demonstrated a number of advantages, such as the absence of cross-over resonances in the SRD spectrum, the simplicity of realization, a low required power, etc. An SRD frequency marker also operates in the presence of the Ne buffer gas at a pressure of 6 Torr, which allowed us to determine the Ne–Rb collisional broadening, whereas the SA method is already inapplicable at buffer gas pressures above 0.1 Torr. The realization simplicity makes the SRD method a convenient tool for atomic spectroscopy. Our theoretical model well describes the SRD signal. more...

Using accurate ab initio calculations of the interaction forces, we employ a quantum mechanical description of the collisional state-changing processes that occur in a cold ion trap with He as a buffer gas. We generate the corresponding inelastic rates for rotational transitions involving three simple molecular anions OH-(1Σ), MgH-(1Σ), and C2H-(1Σ) colliding with the helium atoms of the trap. We show that the rotational constants of these molecular anions are such that within the low-temperature regimes of a cold ion trap (up to about 50 K), a different proportion of molecular states are significantly populated when loading helium as a buffer gas in the trap. By varying the trap operating conditions, population equilibrium at the relevant range of temperatures is reached within different time scales. In the modeling of the photodetachment experiments, we analyze the effects of varying the chosen values for photodetachment rates as well as the laser photon fluxes. Additionally, the changing of the collision dynamics under different buffer gas densities is examined and the best operating conditions, for the different anions, for yielding higher populations of specific rotational states within the ion traps are extracted. The present modeling thus illustrates possible preparation of the trap conditions for carrying out more efficiently state-selected experiments with the trapped anions. more...

This paper presents a comprehensive study of the design and parametric investigation of a diode pump alkali laser (DPAL): an emerging, high quantum efficiency, challenging laser system in the near-infrared region with potential for numerous applications. It covers the design and development of a diode pumped alkali (rubidium) laser (Rb:DPAL) gain cell and associated subsystems including the glove box-based alkali transfer set-up. Laser generation ( $$\lambda {\sim } 795\hbox { nm}$$ ) in a continuous wave (CW) mode with end-pumped geometry in the Rb cell is reported. Comparative studies on the Rb-DPAL output were carried out with various critical operating parameters such as buffer gas pressure, buffer gas composition, gain medium temperature and pump power / intensity. The experimental results were analysed and discussed in light of the efficacy of the pump beam absorption, the laser level kinetics and effective lasing volume in the Rb-DPAL gain cell. more...

We report the hyperfine splitting frequency of ground-state lithium atoms. The frequency shift is induced by the collisions to helium and xenon buffer gases. Coherent population-trapping resonance between the hyperfine levels enables the frequency measurements in the range of pressure from 2 to $$54\,\hbox {kPa}$$ and temperature from 260 to $$400\,^{\circ }\hbox {C}$$ in borosilicate-glass cells. The buffer-gas density is self-consistently calibrated from the hyperfine splitting in the sealed cells. The resonance frequency and the line width provide an insight into collision-induced spin dynamics, where hyperfine-shift, spin-rotation, and spin-exchange interactions play an important role in atomic lithium vapor and buffer gas. more...

Rotational spectra have been measured and assigned for four conformers of trans-urocanic acid. The acid was transferred into the gas phase through laser vaporisation of a solid sample, mixed with a neon buffer gas and then cooled through supersonic expansion. Molecules and complexes in the expanding gas jet were probed through chirped-pulse, Fourier transform microwave spectroscopy between 2.0 and 18.5 GHz. Rotational constants, A0, B0 and C0; centrifugal distortion constants, ΔJ and ΔJK; and nuclear quadrupole coupling constants of the nitrogen atoms, χaa(N) and χbb(N)–χcc(N), were determined for the various conformers. Data were obtained for ten isotopologues of the conformer that was observed to yield the spectrum of highest intensity. Substitution (rs) coordinates were determined for all carbon atoms and two hydrogen atoms of this conformer. Other observed spectra were assigned to conformers on the basis of excellent agreement between calculated and experimentally-determined rotational constants, and empirical observations of the relative intensities of a- and b-type transitions. The results of DFT calculations imply high barriers to the interconversion of assigned conformers. more...

The pulse regime of vaporization of a bulk metal located in a buffer gas is analyzed as a method of generation of metal atoms under the action of a plasma torch or a laser beam. Subsequently these atoms are transformed into solid nanoclusters, fractal aggregates and then into fractal fibers if the growth process proceeds in an external electric field. We are guided by metals in which transitions between s and d-electrons of their atoms are possible, since these metals are used as catalysts and filters in interaction with gas flows. The resistance of metal fractal structures to a gas flow is evaluated that allows one to find optimal parameters of a fractal structure for gas flow propagation through it. The thermal regime of interaction between a plasma pulse or a laser beam and a metal surface is analyzed. It is shown that the basic energy from an external source is consumed on a bulk metal heating, and the efficiency of atom evaporation from the metal surface, that is the ratio of energy fluxes for vaporization and heating, is 10–3–10–4 for transient metals under consideration. A typical energy flux (~106 W/cm2), a typical surface temperature (~3000 K), and a typical pulse duration (~1 μs) provide a sufficient amount of evaporated atoms to generate fractal fibers such that each molecule of a gas flow collides with the skeleton of fractal fibers many times. more...

We report the detection of the ethynyl (C2H) radical by (1 + 2) REMPI on the B 2 A ′ ( T , K a = 0 , 1 , 2 ) ← X 2 Σ + ( 0 , 3 1 , 0 ) transition. One parallel, B 2 A ′ ( T , K a = 1 ) ← X 2 Σ + ( 0 , 3 1 , 0 ) with E eV = 37 , 946 cm - 1 , and two perpendicular transitions, B 2 A ′ ( T , K a = 0 ) ← X 2 Σ + ( 0 , 3 1 , 0 ) with E eV = 37 , 927 cm - 1 and B 2 A ′ ( T , K a = 2 ) ← X 2 Σ + ( 0 , 3 1 , 0 ) with E eV = 38 , 002 cm - 1 , are observed. The C2H radical was generated by the photodissociation of C2H2 at 193 nm. By employing REMPI spectroscopy, we also studied the collisional induced rotational relaxation of C2H and measured rotational relaxation rate constants for this radical. The measured rotational relaxation rate constants for several values of J″ are ( 0.08 – 0.26 ) × 10 - 9 cm 3 molec - 1 s - 1 and ( 0.75 – 1.92 ) × 10 - 9 cm 3 molec - 1 s - 1 for nitrogen buffer gas and acetylene, respectively. more...

IR diode laser spectroscopy was used to detect the products of HCNO (fulminic acid) photolysis at 193 nm. Six product channels are energetically possible at this photolysis wavelength: O + HCN, H + NCO/CNO, CN + OH, CO + NH, NO + CH and HNCO. In some experiments, isotopically labeled 15 N 18 O, C 2 D 6 or C 6 H 12 reagents were included into the photolysis mixture in order to suppress and/or redirect possible secondary reactions. HCN, OC 18 O, 15 N 15 NO, CO, DCN and HNCO molecules were detected upon laser photolysis of HCNO/reagents/buffer gas mixtures. Analysis of the yields of product molecules leads to the following photolysis quantum yields: ϕ 1a (O + HCN) = 0.38 ± 0.04, ϕ 1b (H + (NCO)) = 0.07 ± 0.02, ϕ 1c (CN + OH) = 0.24 ± 0.03, ϕ 1d (CO + NH(a 1 Δ)) ϕ 1e (HNCO) = 0.02 ± 0.01 and ϕ 1f (CH + NO) = 0.21 ± 0.1, respectively. more...

High temperatures and associated poor rotational partition functions have hindered the application of high-resolution rotational spectroscopy for detection of molecules desorbing from an ice substrate. Here, an experimental approach is presented which will enable such investigations through incorporation of a buffer gas cooling cell. We discuss design considerations for this apparatus and the expected performance based upon OC34S measurements made with a related spectrometer in a 14.7 K sampled uniform flow. We highlight specific applications of this technique including chiral sensing of molecules generated in an interstellar ice, and the synthesis of exotic species not amenable to generation in the gas-phase. more...

In this work, a novel diagnostic technique for carbon monoxide (CO) number density measurements in a nitrogen buffer mixture at elevated pressures up to 5 bar was developed and tested. The technique utilizes 2 + 1 resonance enhanced multi-photon ionization (REMPI) of CO induced by a femtosecond laser pulse at 230.1 nm, followed by detection of the number of REMPI-induced electrons using the microwave scattering (MS) method (REMPI-MS technique). Dependences of the number of REMPI-generated electrons on CO number density and laser energy were measured and analyzed in conjunction with a four energy level model of the CO molecule. The number of REMPI-induced electrons scaled linearly with CO number density up to about 5 × 1018 cm−3 and was independent of the buffer gas pressure up to 5 bar. Higher CO number densities caused saturation onset associated with laser beam energy loss while travelling through the gaseous mixture due to two-photon absorption and photoionization. The number of REMPI-induced electrons was found to scale cubically with the laser pulse energy for the tested energy range of 8–20 μJ (intensity in the focal region about 7–18 GW/cm2), which is consistent with the operation regime where the number density of excited CO molecules increases throughout the laser pulse duration and does not saturate in time. The linear scaling region of the REMPI-MS signal can be used for a CO number density diagnostic after appropriate calibration of the system. more...

A novel two-stage helium buffer gas cooled beam source is introduced. The properties of the molecular beams produced from this source are investigated theoretically using the CaF as a test molecule. The gas-phase molecules are first produced inside a 3 K helium buffer gas cell by laser ablation and subsequently cooled down to 3 K by collisions with buffer gas atoms. The precooled molecules are then extracted into the 0.5 K helium buffer gas cell where they are cooled further down to 0.5 K by collisions with cold helium atoms. Finally, the cold molecules are extracted out into the high vacuum through the 0.5 K cell exit aperture and form a molecular beam. The mean forward velocity and extraction efficiency of the molecules from the buffer gas cells into the molecular beam are calculated to be 67 m/s and 16 % respectively when both buffer gas cells are operated in hydrodynamic entrainment regime. Benefitted from this high extraction efficiency as well as the increased number of slower molecules resulting from the low operating temperature of the second stage cell, this source will enable efficient magneto-optical trapping of the molecules, investigations of sympathetic cooling of the molecules with ultracold atoms, and performing ultrahigh precision molecular spectroscopy. more...

Cooling of trapped ions with a neutral buffer gas makes the study of atom–ion hybrid systems possible in the quantum regime. The new record low achieved opens the door to numerous opportunities, including full control over the atom–ion interactions. more...

We describe an optically pumped Cs magnetometer containing a 1.5 mm thick microfabricated vapor cell with nitrogen buffer gas operating in a free-induction-decay (FID) configuration. This allows us to monitor the free Larmor precession of the spin coherent Cs atoms by separating the pump and probe phases in the time domain. A single light pulse can sufficiently polarize the atomic sample however, synchronous modulation of the light field actively drives the precession and maximizes the induced spin coherence. Both amplitude and frequency modulation have been implemented with noise floors of 3 pT / √ Hz and 16 pT / √ Hz respectively within the Nyquist limited bandwidth of 500 Hz . more...

We have developed a method to quantify reaction product ratios using high resolution microwave spectroscopy in a cryogenic buffer gas cell. We demonstrate the power of this method with the study of the ozonolysis of isoprene, CH2[double bond, length as m-dash]C(CH3)-CH[double bond, length as m-dash]CH2, the most abundant, non-methane hydrocarbon emitted into the atmosphere by vegetation. Isoprene is an asymmetric diene, and reacts with O3 at the 1,2 position to produce methyl vinyl ketone (MVK), formaldehyde, and a pair of carbonyl oxides: [CH3CO-CH[double bond, length as m-dash]CH2 + CH2[double bond, length as m-dash]OO] + [CH2[double bond, length as m-dash]O + CH3COO-CH[double bond, length as m-dash]CH2]. Alternatively, O3 could attack at the 3,4 position to produce methacrolein (MACR), formaldehyde, and two carbonyl oxides [CH2[double bond, length as m-dash]C(CH3)-CHO + CH2[double bond, length as m-dash]OO] + [CH2[double bond, length as m-dash]O + CH2[double bond, length as m-dash]C(CH3)-CHOO]. Purified O3 and isoprene were mixed for approximately 10 seconds under dilute (1.5-4% in argon) continuous flow conditions in an alumina tube held at 298 K and 5 Torr. Products exiting the tube were rapidly slowed and cooled within the buffer gas cell by collisions with cryogenic (4-7 K) He. High resolution chirped pulse microwave detection between 12 and 26 GHz was used to achieve highly sensitive (ppb scale), isomer-specific product quantification. We observed a ratio of MACR to MVK of 2.1 ± 0.4 under 1 : 1 ozone to isoprene conditions and 2.1 ± 0.2 under 2 : 1 ozone to isoprene conditions, a finding which is consistent with previous experimental results. Additionally, we discuss relative quantities of formic acid (HCOOH), an isomer of CH2[double bond, length as m-dash]OO, and formaldehyde (CH2[double bond, length as m-dash]O) under varying experimental conditions, and characterize the spectroscopic parameters of the singly-substituted 13C trans-isoprene and 13C anti-periplanar-methacrolein species. This work has the potential to be extended towards a complete branching ratio analysis, as well towards the ability to isolate, identify, and quantify new reactive intermediates in the ozonolysis of alkenes. more...

The kinetics of ignition in C2H2:O2:Ar, C2H4:O2:Ar, C2H6:O2:Ar and C2H5OH:O2:Ar mixtures was experimentally analyzed at temperatures above the self-ignition threshold using a shock tube with a discharge cell. For this purpose, ignition delay time was measured in lean and stoichiometric C2H4-containing mixtures and lean C2H6-containig mixtures behind a reflected shock wave after a high-voltage nanosecond discharge and in its absence. Obtained results were compared with previous studies of C2-hydrocarbon plasma ignition conducted with the same setup. Numerical simulations of the discharge and ignition phases were used to show the main mechanisms that control ignition properties of C2-hydrocarbons. The densities of atoms, radicals, excited and charged particles produced in the discharge plasma were calculated and used as input parameters for ignition modeling. Good agreement was obtained between calculated and measured ignition delay times in the mixtures studied with and without the discharge. Sensitivity analysis was used to demonstrate the limiting processes in the discharge and ignition phases. It was shown that electron interaction with hydrocarbon molecules can affect ignition directly, due to additional dissociation, or indirectly, due to additional electron-impact ionization and the increase of energy deposited in the discharge. The first effect is more pronounced in C2H2 and C2H4 ignition in the stoichiometric mixtures, whereas the second effect is more profound for ignition in the C2H5OH-containing mixtures. The comparison of plasma-assisted ignition dynamics for different C2-hydrocarbons leads to the conclusion that the efficiency of nonequilibrium excitation for ignition and combustion control strongly depends on the type of fuel. The fuels can affect the total discharge energy, the efficiency of hydrogen atom and hydrocarbon radical production in the discharge, and the dynamics of oxygen molecule dissociation by quenching of electronically-excited states of buffer gas. more...

It is known that the ion collision cross section (CCS) may be calculated from the linewidth of a Fourier transform ion cyclotron resonance (FT-ICR) mass spectral peak at elevated pressure (e.g., ∼10(-6) Torr). However, the high mass resolution of FT-ICR is sacrificed in those experiments due to high buffer gas pressure. In this study, we describe a linewidth correction method to eliminate the windowing-induced peak broadening effect. Together with the energetic ion-neutral collision model previously developed by our group, this method enables the extraction of CCSs of biomolecules from high-resolution FT-ICR mass spectral linewidths, obtained at a typical operating buffer gas pressure of modern FT-ICR instruments (∼10(-10) Torr). CCS values of peptides including MRFA, angiotensin I, and bradykinin measured by the proposed method agree well with ion mobility measurements, and the unfolding of protein ions (ubiquitin) at higher charge states is also observed. more...

We demonstrate continuous-wave cavity ring-down spectroscopy of a partially hydrodynamic molecular beam emerging from a buffer-gas-cooling source. Specifically, the ( v1 + v(3)) vibrational overtone band of acetylene (C2H2) around 1.5 mu m is accessed using a narrow-linewidth diode laser stabilized against a GPS-disciplined rubidium clock via an optical frequency comb synthesizer. As an example, the absolute frequency of the R(1) component is measured with a fractional accuracy of 1 X 10(-9). Our approach represents the first step towards the extension of more sophisticated cavity-enhanced interrogation schemes, including saturated absorption cavity ring-down or two-photon excitation, to buffer-gascooled molecular beams. more...

Processes are considered for conversion into a fractal structure of a hot metal micron-size particle that is located in a buffer gas or a gas flow and is heated by an external electric or electromagnetic source or by a plasma. The parameter of this heating is the particle temperature, which is the same in the entire particle volume because of its small size and high conductivity. Three processes determine the particle heat balance: particle radiation, evaporation of metal atoms from the particle surface, and heat transport to the surrounding gas due to its thermal conductivity. The particle heat balance is analyzed based on these processes, which are analogous to those for bulk metals with the small particle size, and its high temperature taken into account. Outside the particle, where the gas temperature is lower than on its surface, the formed metal vapor in a buffer gas flow is converted into clusters. Clusters grow as a result of coagulation until they become liquid, and then clusters form fractal aggregates if they are removed form the gas flow. Subsequently, associations of fractal aggregates join into a fractal structure. The rate of this process increases in medium electric fields, and the formed fractal structure has features of aerogels and fractal fibers. As a result of a chain of the above processes, a porous metal film may be manufactured for use as a filter or catalyst for gas flows. more...

A force field molecular dynamics method is developed to directly simulate ion drift in buffer gases driven by an electric field. The ion mobility and collision cross sections (CCSs) with relevance to ion mobility spectrometry can be obtained from the simulated drift velocity in high-density buffer gases (pressure ∼50 bars) and high electric fields (∼107 V/m). Compared to trajectory methods, the advantage of the molecular dynamics method is that it can simultaneously sample the internal dynamic motions of the ion and the ion-gas collisions. For ions with less than 100 atoms, the simulated collision cross section values can be converged to within ±1%-2% by running a 100 ns simulation for 5-19 h using one computer core. By using a set of element-based Lennard-Jones parameters that are not tuned for different atomic types in different molecules, the simulated collision cross sections for 15 small molecular ions (number of atoms ranging from 17 to 85, mass ranging from 74.1 to 609.4 g/mol) are consistent with experimental values: the mean unsigned error is 2.6 A2 for He buffer gas and 4.4 A2 for N2 buffer gas. The sensitivity of the simulated CCS values to random diffusion, drift velocity, electric field strength, temperature, and buffer gas density is examined. more...

A systematic study of laser-Induced thermal grating scattering (LITGS) using nitric oxide as an absorbing species is presented as a means of thermometry in air-fed combustion. The relative contributions to the scattered signal from degenerate four-wave mixing, DFWM, and from laser induced thermal grating scattering, LITGS, are studied in the time domain for NO in N2 buffer gas up to 4 bar, using a pulsed laser system to excite the (0,0) -bands of NO at 226.21 nm. LITGS signals from combustion-generated NO in a laminar, pre-mixed CH4/O2/N2 flame on an in-house constructed slot burner were used to derive temperature values as a function of O2 concentration and position in the flame at 1 bar and 2.5 bar total pressure. Temperature values consistent with the calculated adiabatic flame temperature were derived from averaged LITGS signals over 50 – 100 single shots at 10 Hz repetition rate in the range 1600 K – 2400 K with a pressure-dependent uncertainty of ±1.8% at 1 bar to ±1.4% at 2.5 bar. Based on observed signal-to-noise ratios the minimum detectable concentration of NO in the flame is estimated to be 80 ppm for a 5 second measurement time at 10 Hz repetition rate. more...

We demonstrated sympathetic cooling of a single ion in a buffer gas of ultracold atoms with small mass. Efficient collisional cooling was realized by suppressing collision-induced heating. We attempt to explain the experimental results with a simple rate equation model and provide a quantitative discussion of the cooling efficiency per collision. The knowledge we obtained in this work is an important ingredient for advancing the technique of sympathetic cooling of ions with neutral atoms., Comment: 5 pages, 3 figures, published in Phys. Rev. Lett 120, 043401 (2018) more...

We present an extensive range of accurate ab initio calculations, which map in detail the spatial electronic potential energy surface that describes the interaction between the molecular anion NH2- ((1)A(1)) in its ground electronic state and the He atom. The time-independent close-coupling method is employed to generate the corresponding rotationally inelastic cross sections, and then the state-changing rates over a range of temperatures from 10 to 30 K, which is expected to realistically represent the experimental trapping conditions for this ion in a radio frequency ion trap filled with helium buffer gas. The overall evolutionary kinetics of the rotational level population involving the molecular anion in the cold trap is also modelled during a photodetachment experiment and analyzed using the computed rates. The present results clearly indicate the possibility of selectively detecting differences in behavior between the ortho- and para-anions undergoing photodetachment in the trap., Austrian Science Fund (FWF); Computational Center of the University of Innsbruck more...

One of the most important atomic properties governing an element's chemical behavior is the energy required to remove its least-bound electron, referred to as the first ionization potential. For the heaviest elements, this fundamental quantity is strongly influenced by relativistic effects which lead to unique chemical properties. Laser spectroscopy on an atom-at-a-time scale was developed and applied to probe the optical spectrum of neutral nobelium near the ionization threshold. The first ionization potential of nobelium is determined here with a very high precision from the convergence of measured Rydberg series to be 6.626 21±0.000 05 eV. This work provides a stringent benchmark for state-of-the-art many-body atomic modeling that considers relativistic and quantum electrodynamic effects and paves the way for high-precision measurements of atomic properties of elements only available from heavy-ion accelerator facilities. ispartof: PHYSICAL REVIEW LETTERS vol:120 issue:26 pages:263003-263003 ispartof: location:United States status: published more...

Many reports on stimulated Raman scattering in mixtures of Raman-active and noble gases indicate that the addition of a dispersive buffer gas increases the phase mismatch to higher-order Stokes and anti-Stokes sidebands, resulting in a preferential conversion to the first few Stokes lines, accompanied by a significant reduction in the Raman gain due to collisions with gas molecules. Here we report that, provided the dispersion can be precisely controlled, the effective Raman gain in a gas-filled hollow-core photonic crystal fiber can actually be significantly enhanced when a buffer gas is added. This counterintuitive behavior occurs when the nonlinear coupling between the interacting fields is strong and can result in a performance similar to that of a pure Raman-active gas, but at a much lower total gas pressure, allowing competing effects such as Raman backscattering to be suppressed. We report high modal purity in all the emitted sidebands, along with anti-Stokes conversion efficiencies as high as 5% in the visible and 2% in the ultraviolet. This new class of gas-based waveguide device, which allows the nonlinear optical response to be beneficially pressure-tuned by the addition of buffer gases, may find important applications in laser science and spectroscopy. more...

We use narrow-band laser excitation of Yb atoms to substantially enhance the brightness of a cold beam of YbOH, a polyatomic molecule with high sensitivity to physics beyond the standard model (BSM). By exciting atomic Yb to the metastable 3P1 state in a cryogenic environment, we significantly increase the chemical reaction cross-section for collisions of Yb with reactants. We characterize the dependence of the enhancement on the properties of the laser light, and study the final state distribution of the YbOH products. The resulting bright, cold YbOH beam can be used to increase the statistical sensitivity in searches for new physics utilizing YbOH, such as electron electric dipole moment and nuclear magnetic quadrupole moment experiments. We also perform new quantum chemical calculations that confirm the enhanced reactivity observed in our experiment and compare reaction pathways of Yb(3P) with the reactants H2O and H2O2. More generally, our work presents a broad approach for improving experiments that use cryogenic molecular beams for laser cooling and precision measurement searches of BSM physics. more...

Recombination of N2H+ ions with electrons was studied using a stationary afterglow with a cavity ring-down spectrometer. We probed in situ the time evolutions of number densities of different rotational and vibrational states of recombining N2H+ ions and determined the thermal recombination rate coefficients for N2H+ in the temperature range of 80–350 K. The newly calculated vibrational transition moments of N2H+ are used to explain the different values of recombination rate coefficients obtained in some of the previous studies. No statistically significant dependence of the measured recombination rate coefficient on the buffer gas number density was observed.Recombination of N2H+ ions with electrons was studied using a stationary afterglow with a cavity ring-down spectrometer. We probed in situ the time evolutions of number densities of different rotational and vibrational states of recombining N2H+ ions and determined the thermal recombination rate coefficients for N2H+ in the temperature range of 80–350 K. The newly calculated vibrational transition moments of N2H+ are used to explain the different values of recombination rate coefficients obtained in some of the previous studies. No statistically significant dependence of the measured recombination rate coefficient on the buffer gas number density was observed. more...

In this paper, the spin dynamics and broadening mechanism of cesium vapor in cells without buffer gas is investigated by means of spin noise spectroscopy. In a macro atomic vapor cell, the lineshape of detuning frequency spectrum of spin relaxation rate is of Gaussian distribution. For a micron-scaled vapor cell with strong spatial locality, the lineshape of detuning frequency spectrum of spin relaxation rate is of Lorentzian distribution. The parameter dependence of detuning frequency spectrum of spin relaxation rate, such as temperature, is studied quantitatively. The detuning frequency spectrum of the spin relaxation rate is measured experimentally to be broadened by \begin{document}$ \sim $\end{document}4 GHz, which is obviously larger than the unhomogeneous Doppler broadening of \begin{document}$ \sim $\end{document}500 MHz for a macro atomic vapor cell. At the same time, the detuning frequency spectrum of total noise in the two atomic vapor cells is studied. In the macro atomic vapor cell, the total noise intensity strongly relies on the detuning frequency of the laser with respect to the atomic resonance transition. In the micron-scaled vapor cell, due to the strong homogeneous broadening, the center of the detuning frequency spectrum of the total noise is observed to dip. Finally, a simplified physical model is established to compute the broadening of the micron-scaled vapor cell. The homogeneous broadening of atoms is explained experimentally and theoretically in the micron-scaled vapor cell. more...

Diode-pumped alkali lasers (DPALs) have undergone rapid development to become one of the most promising candidates for use as high-power laser sources in recent years. Relaxation oscillation (RO) is a common phenomenon related to the dynamic process in the time domain. Sometimes, it is applied in parameter measurement, but sometimes it should be eliminated to ensure stable output. In this paper, we develop a kinetic model to study the RO features of a DPAL, which are different from those of a conventional solid-state laser. The results reveal that the cell temperature, buffer gas pressure, pumping power, cavity length, and reflectance of an output coupler affect the characteristics of ROs. Among these parameters, the cell temperature and the pumping power exert relatively strong influences on the waiting time of the first spike in the RO. Additionally, the cavity length cannot markedly affect the peak value of the laser intensity. These new analyses should prove useful for understanding the dynamic process of DPAL oscillation and for the future design of a steady high-powered DPAL. more...

Experimental and numerical studies have been performed on the thermal and chemical effects of buffer gas composition on low temperature ignition of iso-octane and n-heptane. Experiments were conducted using a recently developed rapid compression machine in the temperature range of 600–850 K. Three buffer gases were studied including nitrogen (N 2 ), argon (Ar), and a mixture of Ar and carbon dioxide (CO 2 ) at a mole ratio of 65.1%/34.9%. Iso-octane was studied at 20 bar, ϕ = 1, and a dilution level of buffer gas to O 2 of 3.76:1 (mole ratio). n-Heptane was studied at 9 bar, ϕ = 1, and a dilution level of buffer gas to O 2 of 5.63:1 (mole ratio). For experiments where two-stage ignition was observed, the buffer gas composition had no impact on the first-stage ignition time but, as expected, it caused differences in the total heat release, pressure and temperature rise after the first-stage ignition. As a consequence, significant differences were observed for the total ignition delay time as a function of the buffer gas composition, with up to 40% and 42.5% faster total ignition time for iso-octane and n-heptane, respectively, by using Ar instead of N 2 . The chemical effects of the buffer gas composition were studied experimentally by comparing the results of the N 2 and Ar/CO 2 (65.1%/34.9%) mixtures, recognizing that while the Ar/CO 2 mixture has the same heat capacity as N 2 , its predicted combined third-body collision efficiency is about 76% higher than N 2 . The experimental results showed negligible chemical effects on the first-stage and total ignition delay times. Numerical simulations were carried out over a wider range of temperatures for pure N 2 , Ar, and CO 2 as buffer gases. Results showed that thermal effects are very pronounced and dominated at the negative temperature coefficient and two-stage ignition conditions, which is consistent with the experimental results and previous studies in the literature. However, the simulation results also showed at temperatures higher than 850 K, the chemical effects of CO 2 became more important than the thermal effects. more...

The operation of a transversely diode-pumped alkali metal vapor laser amplifier is theoretically studied. The amplifier operation is described by a rather intricate system of differential equations, which can be solved in the general case only numerically. In the case of intense incident radiation, an analytic solution is obtained which makes it possible to determine any energy characteristics of the laser amplifier and to find the optimal parameters of the active medium and pump radiation (temperature, buffer gas pressure, and intensity and width of the pump radiation spectrum). more...

The intrinsic relaxation rates of the vector magnetization of cesium vapor enclosed in microfabricated atomic magnetometer cells are investigated. Two methods—the optically detected magnetic resonance and the ground-state Hanle effect—are used to carry out automated measurements in dependence on cell temperature and nitrogen buffer gas pressure. The experimental results are compared with expected contributions of the different relaxation processes and in this way allow the discrimination between them to help further optimization of cell design. The methods are compared in terms of basic features, data quality, and practical applicability. more...

This paper presents an experimental investigation on the ignition characteristics of laser-ionized fuel injected into a hypersonic air-crossflow. A Q-switched laser causes breakdown in the sonic H 2 injector stream before interaction with the air-crossflow traveling at 2 km/s with a total specific enthalpy of 2.5 MJ/kg. The flowfield is visualized using schlieren imaging and planar laser-induced fluorescence (PLIF) on the NO molecule. The temporal evolution of the ignition process is visualized using PLIF on the OH molecule. We compare the ignition effectiveness, in terms of the OH PLIF signal, between a series of test cases with different injector pressures and laser energies. These results are also compared to a previous study, where the laser-spark was generated in the jet interaction region, outside the fuel injector. The influence of using H 2 fuel diluted with Ar, which serves as a plasma buffer gas, to extend plasma lifetimes is also investigated. The ignition technique is found to be effective in terms of post-plasma hydroxyl production, with a significant increase in performance observed when Ar is used as a plasma buffer gas. more...

This work quantifies the chemical kinetic and thermal effects of buffer gas composition on autoignition of three fuels at conditions relevant to engines, combustors, and experimental facilities used to study ignition kinetics. Computational simulations of autoignition of iso-octane, n-heptane, and of n-butanol were used to characterize the effects of buffer gas composition on ignition delay time and heat release rate. Stoichiometric mixtures, ϕ = 1.0, and a temperature range of 600–1100 K were considered. Iso-octane and n-heptane were studied at initial pressures of 9.0 atm and 60.0 atm, and n-butanol was studied at initial pressures of 3.2 atm and 60.0 atm. Two dilution levels of buffer gas to O2 of 3.76:1 (mole basis) and 5.64:1 were considered (∼21% and ∼15% O2 respectively, mole basis). The fuels and simulation conditions were selected based on the relevance to engine operating conditions and previously published ignition studies. The buffer gases considered were argon, nitrogen, water, and carbon dioxide. Simulation results predicted changes of greater than a factor of 2 in ignition delay time and heat release rate as a function of buffer gas composition in the negative temperature coefficient (NTC) region for n-heptane and iso-octane. Outside the NTC region, the predicted effects of changes in buffer gas composition were small ( a factor of 2). The heat release rates were also sensitive to buffer gas composition, with carbon dioxide exhibiting relatively high levels of early and late heat release relative to the other buffer gases. Sensitivity analysis of the third-body collision efficiencies for the buffer gases showed the effects of uncertainties in the third body collision efficiencies on ignition delay time and heat release rate. The results highlight the significance of buffer gas composition on low-temperature combustion chemistry, particularly via H2O2 and HO2 decomposition and recombination reactions. more...

The rotational spectra of N-, 2-, 4-, and 5-methylimidazole are reported and analyzed. Liquid N-methylimidazole was vaporized from a reservoir, and each of 2-, 4-, and 5-methylimidazole was laser-vaporized from a solid target prior to mixing with argon buffer gas and undergoing supersonic expansion from a pulsed nozzle. The spectra were recorded by chirped-pulse Fourier transform microwave spectroscopy in the 7.0-18.5 GHz frequency range. Rotational constants, A0, B0, and C0, centrifugal distortion constants, DJ, DJK, DK, d1, and d2, and nuclear quadrupole coupling constants of nitrogen atoms, χaa(N1), χbb(N1) - χcc(N1), χaa(N3), and χbb(N3) - χcc(N3), are determined from experimentally measured transition frequencies. Data recorded for isotopologues containing 13C or 15N are used to determine the rs coordinates of all heavy atoms in N-, 2-, and 4-methylimidazole. The results allow fitting of parameters in the Hamiltonian that describes internal rotation of the CH3 group about its local C3 axis. The V3 terms in the periodic potential energy functions that describe the internal rotation in N-, 2-, 4-, and 5-methylimidazole are determined to be 185.104(11), 122.7529(38), 317.20(14), and 386.001(19) cm-1, respectively, by the internal axis method. The experiments are supported by density functional theory calculations. Observed variations in barrier height are explained with reference to the symmetry of overlap between a π-like orbital on the CH3 group and π-orbitals on the aromatic ring. more...