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The rotationally resolved infrared (IR) spectrum of the He-H

With a He-H

© 2020 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim In order to study collisions between ions and neutrals, a new Guided Ion Beam (GIB) apparatus, called NOVion, has been assembled and tested. The primary purpose of this instrument is to measure absolute cross sections at energies relevant for technical or inter- and circumstellar plasmas. New and improved results are presented for forming H3+ in collisions of H2+ with H2. Between 0.1 eV and 2 eV, our measured effective cross sections are in good overall agreement with most previous measurements. However, at higher energies, our results do not show the steep decline, recommended in the standard literature. After critical evaluation of all experimental and theoretical data, a new analytical function is proposed, describing properly the dependence of the title reaction on the collision energy up to 10 eV.

The rotationally resolved infrared (IR) spectrum of the He–[Formula: see text] complex has been measured in a cryogenic ion trap experiment at a nominal temperature of 4 K. Predissociation of the stored complex has been invoked by excitation of the degenerate ν2 mode of the [Formula: see text] sub-unit using a pulsed optical parametric oscillator system. An assignment of the experimental spectrum became possible through one-to-one correlations with bands of the spectrum theoretically predicted in Paper I [Harding et al., J. Chem. Phys. 156, 144307 (2022)]. 19 bands have been assigned and analyzed, and the energy term diagram of the lower states of this floppy molecular complex has been derived from combination differences (CDs) in the experimental spectrum. Ground state combination differences (GSCDs) reveal a large part of the energy term diagram for the He–[Formula: see text] complex in its vibrational ground state, v = 0. Experimental and theoretical term energies agree within experimental accuracy for the rotational fine structure associated with the total angular momentum quantum number J and the parity e/ f as well as for the coarse spacing of the lowest K states of the complex. This favorable comparison shows that the potential energy surface (PES) calculated in Paper I is accurate. The barriers between the three equivalent global minima in this PES are relatively low and the He–[Formula: see text] complex is extremely floppy, with nearly unhindered internal rotation of the [Formula: see text] sub-unit. The resulting Coriolis interactions couple the internal and end-over-end rotation of the complex and contribute significantly to the energy terms. They are observed both in experiment and theory and are, e.g., the origin of different rotational constants for states of e and f parity. Also in this respect, experiment and theory agree very well. Despite the assignment and analysis of many bands of the extremely rich IR spectrum of He–[Formula: see text], higher levels of excitation, including the complex stretching mode, need further attention.

With a He–[Formula: see text] interaction potential obtained from advanced electronic structure calculations, we computed the vibration-rotation-tunneling (VRT) states of this complex for total angular momenta J from 0 to 9, both for the vibrational ground state and for the twofold degenerate v2 = 1 excited state of [Formula: see text]. The potential has three equivalent global minima with depth D e = 455.3 cm−1 for He in the plane of [Formula: see text], three equatorial saddle points that separate these minima with barriers of 159.5 cm−1, and two axial saddle points with energies of 243.1 cm−1 above the minima. The dissociation energies calculated for the complexes of He with ortho-[Formula: see text] (o[Formula: see text]) and para-[Formula: see text] (p[Formula: see text]) are D0 = 234.5 and 236.3 cm−1, respectively. Wave function plots of the VRT states show that they may be characterized as weakly hindered internal rotor states, delocalized over the three minima in the potential and with considerable amplitude at the barriers. Most of them are dominated by the j k = 10 and 11 rotational ground states of o[Formula: see text] and p[Formula: see text], with the intermolecular stretching mode excited up to v = 4 inclusive. However, we also found excited internal rotor states: 33 in He–o[Formula: see text], and 22 and 21 in He–p[Formula: see text]. The VRT levels and wave functions were used to calculate the frequencies and line strengths of all allowed v2 = 0 → 1 rovibrational transitions in the complex. Theoretical spectra generated with these results are compared with the experimental spectra in Paper II [Salomon et al., J. Chem. Phys. 156, 144308 (2022)] and are extremely helpful in assigning these spectra. This comparison shows that the theoretical energy levels and spectra agree very well with the measured ones, which confirms the high accuracy of our ab initio He–[Formula: see text] interaction potential and of the ensuing calculations of the VRT states.

This work is motivated by many observations of nitrogen hydrides including their isotopologues in the interstellar space. We studied the formation of NH+ and ND+ ions in the nearly thermoneutral hydrogen abstraction reactions of N+ ions with H2, HD, and D2 at temperatures from 300 K down to 15 K using a variable-temperature 22-pole radio frequency ion trap. For the reaction of N+ with HD, the branching ratios for production of ND+ and NH+ ions were also determined. The activation energies of all four reaction channels were determined from the temperature dependencies of the measured reaction rate coefficients. Under the assumption of no energy barriers on the reaction paths, we derive the vibrationless energy change (i.e., the difference of equilibrium Born–Oppenheimer potential energies of products and reactants) in the reactions as ΔE e = (103 ± 3) meV.

Helium tagging of charged C60q+ ions (q = 1–3) has been used for measuring electronic and infrared gas phase spectra, allowing astronomers to improve the identification of fullerenes in space. Here, we present a detailed study of the attachment of He to cold mass selected fullerene ions. Experiments were performed in the temperature variable radio frequency (rf) ion trap ISORI at high He densities and a few K. For all three charge states, the ternary rate coefficients for forming He-C60q+ are below 10−31 cm6s–1 and all three show temperature dependences, proportional to exp(-T/T0) with T0.

In order to study collisions between ions and neutrals, a new Guided Ion Beam (GIB) apparatus, called NOVion, has been assembled and tested. The primary purpose of this instrument is to measure absolute cross sections at energies relevant for technical or inter- and circumstellar plasmas. New and improved results are presented for forming H

This paper reports gas-phase IR spectra of (n = 1–3) ions in the range from 1100 to 1600 cm−1. The spectra were recorded with a temperature-variable radio frequency ion trap instrument. The experiments work with mass-selected ions confined in a linear wire quadrupole, where they form helium-tagged complexes following exposure to low-temperature (as low as 2.3 K) and high-density helium buffer gas. All measured lines are homogeneously broadened owing to a strong coupling between the vibrational states of the fullerene cage. The short lifetimes of picoseconds or below rule out measurements of high-resolution IR spectra. Reproducible sidebands with distances between 5 and 20 cm−1 enrich the spectra. Tentative explanations are based on nonlinear and symmetry-breaking interactions or may be due to the influence of the attached He atom. Comparison of the spectrum with that of reveals shifts up to 6 cm−1, while the IR spectra of and are very similar. With the harmonic potential of the trap the ions are squeezed into the rather narrow laser beam, increasing the sensitivity and simplifying the determination of absolute photoabsorption cross sections. Disregarding shifts of several cm−1, the gas-phase results for and are in agreement with previous matrix results and with theoretical predictions. The astrophysical relevance of our results is briefly mentioned.

The design and first applications of a new tandem mass spectrometer (BerlinTrap) combining an electrospray ion source, a quadrupole mass spectrometer, a cryogenic 22-pole ion trap (4–300 K), and an orthogonal reflectron time-of-flight mass spectrometer are described. The trapped ions are cooled by helium buffer gas cooling. The formation and solvation shell structure of weakly-bound HenH3O+ complexes and the electronic photodissociation spectrum of the protonated amino acid tyrosine are used to calibrate the setup for cooling, tagging, and spectroscopic capabilities. A vibrational temperature below 20 K is inferred for protonated tyrosine. The electronic spectrum of isolated protonated lumichrome, the smallest protonated flavin, is recorded in the visible range and assigned to the most stable N5 isomer by comparison with quantum chemical calculations. These results demonstrate the suitability of the BerlinTrap apparatus for spectroscopy and reactivity studies of small and large (bio-)molecular ions and their clusters.

The combination of cryogenic ion traps with suitable light sources and standard tools of mass spectrometry has led to many innovative applications in previous years. This paper presents the combination of our versatile instrument with a supercontinuum laser for the rapid identification of ions that might be of special interest, e.g. as candidates for diffuse interstellar bands carriers. Using a linear wire quadrupole ion trap at 3 K, routine He-tagging, long irradiation times, and the brilliance and wide spectral range of a crystal fiber laser, mass selected ions have been exposed to spectral fluencies larger than 10 mJ (nm cm2)−1. These conditions result in an unsurpassed sensitivity, allowing us to find out within a few minutes and with nm accuracy, where photo absorption occurs with cross sections above 10−18 cm2. In this contribution, we present a variety of ions, probed between 420 and 720 nm. They have been generated by electron- or electrospray ionization of (polycyclic) aromatic hydrocarbons. For selected candidates, we recorded spectra with higher resolution and in the IR range. The anthracene dication has been selected to present a detailed analysis of our new results., Combining He-tagging in a cryotrap with a supercontinuum laser is an efficient way of identifying candidates for DIBs carriers.

The interrogation of reaction intermediates is key for understanding chemical reactions; however their direct observation and study remains a considerable challenge. Mass spectrometry is one of the most sensitive analytical techniques, and its use to study reaction mixtures is now an established practice. However, the information that can be obtained is limited to elemental analysis and possibly to fragmentation behavior, which is often challenging to analyze. In order to extend the available experimental information, different types of spectroscopy in the infrared and visible region have been combined with mass spectrometry. Spectroscopy of mass selected ions usually utilizes the powerful sensitivity of mass spectrometers, and the absorption of photons is not detected as such but rather translated to mass changes. One approach to accomplish such spectroscopy involves loosely binding a tag to an ion that will be removed by absorption of one photon. We have constructed an ion trapping instrument capable of reaching temperatures that are sufficiently low to enable tagging by helium atoms in situ, thus permitting infrared photodissociation spectroscopy (IRPD) to be carried out. While tagging by larger rare gas atoms, such as neon or argon is also possible, these may cause significant structural changes to small and reactive species, making the use of helium highly beneficial. We discuss the "innocence" of helium as a tag in ion spectroscopy using several case studies. It is shown that helium tagging is effectively innocent when used with benzene dications, not interfering with their structure or IRPD spectrum. We have also provided a case study where we can see that despite its minimal size there are systems where He has a huge effect. A strong influence of the He tagging was shown in the IRPD spectra of HCCl(2+) where large spectral shifts were observed. While the presented systems are rather small, they involve the formation of mixtures of isomers. We have therefore implemented two-color experiments where one laser is employed to selectively deplete a mixture by one (or more) isomer allowing helium tagging IRPD spectra of the remaining isomer(s) to be recorded via the second laser. Our experimental setup, based on a linear wire quadrupole ion trap, allows us to deplete almost 100% of all helium tagged ions in the trap. Using this special feature, we have developed attenuation experiments for determination of absolute photofragmentation cross sections. At the same time, this approach can be used to estimate the representation of isomers in a mixture. The ultimate aim is the routine use of this instrument and technique to study a wide range of reaction intermediates in catalysis. To this end, we present a study of hypervalent iron(IV)-oxo complexes ([(L)Fe(O)(NO3)](+)). We show that we can spectroscopically differentiate iron complexes with S = 1 and S = 2 according to the stretching vibrations of a nitrate counterion.

Near- and mid-IR absorption spectra of endohedral H

Near- and mid-IR absorption spectra of endohedral H2@C60+ have been measured using He-tagging. The samples have been prepared using a "molecular surgery" synthetic approach and were ionized and spectroscopically characterized in the gas phase. In contrast to neutral C60 and H2@C60, the corresponding He-tagged cationic species show distinct spectral differences. Shifts and line splittings in the near- and mid-IR regions indicate the influence of the caged hydrogen molecule on both the electronic ground and excited states. Possible relevance to astronomy is discussed.

Using a cryogenic linear 22-pole rf ion trap, rate coefficients for H/D exchange reactions of OH− with D2 (1) and OD− with H2 (2) have been measured at temperatures between 11 K and 300 K with normal hydrogen. Below 60 K, we obtained k1 = 5.5 × 10−10 cm3 s−1 for the exoergic reaction (1). Upon increasing the temperature above 60 K, the data decrease with a power law, k1(T) ∼ T−2.7, reaching ≈1 × 10−10 cm3 s−1 at 200 K. This observation is tentatively explained with a decrease of the lifetime of the intermediate complex as well as with the assumption that scrambling of the three hydrogen atoms is restricted by the topology of the potential energy surface. The rate coefficient for the endoergic reaction (2) increases with temperature from 12 K up to 300 K, following the Arrhenius equation, k2 = 7.5 × 10−11 exp(−92 K/T) cm3 s−1 over two orders of magnitude. The fitted activation energy, EA-Exp = 7.9 meV, is in perfect accordance with the endothermicity of 24.0 meV, if one accounts for the thermal population of the rotational states of both reactants. The low mean activation energy in comparison with the enthalpy change in the reaction is mainly due to the rotational energy of 14.7 meV contributed by ortho-H2 (J = 1). Nonetheless, one should not ignore the reactivity of pure para-H2 because, according to our model, it already reaches 43% of that of ortho-H2 at 100 K.

Complexes of the triatomic hydrogen ion with helium were synthesised in a low-temperature 22-pole rf ion trap at He number densities of up to 1016 cm−3. Absolute ternary rate coefficients for sequentially attaching He atoms have been determined from the growth of complexes with increasing storage time. The number of helium-tagged ions is significantly reduced when increasing the nominal temperature from 4 to 25 K. Competition between attachment and dissociation via collisions leads to stationary Hen–H+3 (n up to 9) distributions. State-specific excitation of the trapped H+3 ions via IR transitions significantly reduces the formation of complexes. Tuning the laser to Δv2 = 1 transitions in the range of 2726 cm−1 leads to LIICG lines, i.e., to spectra caused by laser-induced inhibition of complex growth. In addition, almost 100 lines have been found between 2700 and 2765 cm−1, which are attributed to laser-induced dissociation of the in situ formed He–H+3 complex ions. These lines are not yet assigned; howe...

Two-color IR–IR isomer selective predissociation spectra of helium-tagged C6H62+ are presented. The dications are generated via electron bombardment of either benzene or 1,3-cyclohexadiene. After mass selection they are injected into a 2.6 K cold ion trap where the presence of a dense He buffer gas not only cools them but also leads to He attachment. The ion ensemble is exposed to one or two intense IR pulses from optical parametric oscillators (OPOs) (1200–3100 cm–1) before it is extracted, mass analyzed, and detected. On the basis of a comparison with theoretical predictions, the resulting spectral features allow us to separate and assign different isomers of C6H62+ dications. Compression of the ion cloud very close to the axis of the linear quadrupole trap and coaxial superposition of well-collimated laser beams results in the fragmentation of almost all helium complexes at specific wavelengths. This unique feature enables us to record fluence-dependent attenuation curves for individual absorption band...

Stabilization of H3(+) collision complexes has been studied at nominal temperatures between 11 and 33 K using a 22-pole radio frequency (rf) ion trap. Apparent binary rate coefficients, k(*) = kr + k3[H2], have been measured for para- and normal-hydrogen at number densities between some 10(11) and 10(14) cm(-3). The state specific rate coefficients extracted for radiative stabilization, kr(T;j), are all below 2 × 10(-16) cm(3) s(-1). There is a slight tendency to decrease with increasing temperature. In contrast to simple expectations, kr(11 K;j) is for j = 0 a factor of 2 smaller than for j = 1. The ternary rate coefficients for p-H2 show a rather steep T-dependence; however, they are increasing with temperature. The state specific ternary rate coefficients, k3(T;j), measured for j = 0 and derived for j = 1 from measurements with n-H2, differ by an order of magnitude. Most of these surprising observations are in disagreement with predictions from standard association models, which are based on statistical assumptions and the separation of complex formation and competition between stabilization and decay. Most probably, the unexpected collision dynamics are due to the fact that, at the low translational energies of the present experiment, only a small number of partial waves participate. This should make exact quantum mechanical calculations of kr feasible. More complex is three-body stabilization, because it occurs on the H5(+) potential energy surface.

A universal method has been developed for measuring spectra of molecular ions in a 22-pole radio frequency trap at 5 K. It is based on laser induced inhibition of complex growth (LIICG). The first successful measurements have been demonstrated on the A2Πu ← X2Σg+ electronic transition with some thousand N2+ ions, helium densities of 1015 cm–3, and storage times of 1 s. The reduction of the number of N2+–He complexes is the result of an interplay between excitation, radiative and collisional cooling, ternary association, and collision induced dissociation, which is explained by a kinetic model.

The nominal temperature range of cryogenic radio-frequency ion traps has recently been extended down to T=2.3 K. Whereas in situ He tagging of mass-selected ions embedded in dense helium buffer gas is becoming common for recording IR spectra through photofragmentation of small and large ions, much less activity is devoted to the field of cold chemistry, which in this contribution means the two orders of magnitude extending from 300 to below 3 K. The importance of this temperature range for understanding the dynamics of bi- and termolecular reactions is illustrated with new results for the time-honored reaction of FeO

Using the combination of a linear 22-pole ion trap (22 PT) and a coaxial beam of hydrogen atoms, H abstraction from CH x + (x=1, 4 and 5) has been studied. The temperature of the trap, T 22 PT, can be varied between 10 K and 300 K. The velocity distribution of the neutral target beam can be changed by cooling the accommodator (T ACC=10–300 K) and using the focusing features of one or two hexapole magnets. The resulting velocity distributions are characterized by time-of-flight measurements. With the same setup, reactions of mass selected stored ions with a cold effusive beam of H2 molecules have been measured with the discharge turned off. At temperatures of interstellar space, H-abstraction from CH+ is efficient. CH4 + reacts five times faster with H than with H2 at 50 K. In contradiction to ab initio calculations and to the established proton affinity of methane, CH5 + is slowly destroyed in collisions with H in our trap, even at 10 K. Some first results for collisions between CH x + and D atoms are reported. For x=1, H–D exchange is quite efficient, even though it is in competition with the exothermic abstraction reaction. For x=4, H-abstraction, i.e. formation of HD molecules, dominates. Deuteration of CH5 + is measured to be very slow.

The neutral molecule temperature dependence of the rate coefficient for the electron transfer reaction from H(2)O to N(2)(+) is determined using a coaxial molecular beam radio frequency ring electrode ion trap (CoMB-RET) method. The temperature of the N(2)(+) ions was maintained at 100 K, while the effusive water beam temperature was varied from 300 to 450 K. The result demonstrates the neutral molecule rotational/translational energy dependence on the rate coefficient of an ion-dipolar molecule reaction. It is found that the rate coefficient in the above temperature range follows the prediction of the simplest ion-dipole capture model. Use of different buffer gas collisional cooling in both the ion source and the RET reveals the effects of both translational and vibrational energy of the N(2)(+) ions.

Combining a temperature variable 22-pole ion trap with a cold effusive beam of neutrals, rate coefficients k ( T ) have been measured for reactions of CO 2 + ions with H, H 2 and deuterated analogues. The neutral beam which is cooled in an accommodator to T ACC , penetrates the trapped ion cloud with a well-characterized velocity distribution. The temperature of the ions, T 22PT , has been set to values between 15 and 300 K. Thermalization is accelerated by using helium buffer gas. For reference, some experiments have been performed with thermal target gas. For this purpose hydrogen is leaked directly into the box surrounding the trap. While collisions of CO 2 + with H 2 lead exclusively to the protonated product HCO 2 + , collisions with H atoms form mainly HCO + . The electron transfer channel H + + CO 2 could not be detected ( 2 + ions nor on the velocity of the atoms ( k ( T ) lays between 4.5 and 4.7 × 10 −10 cm 3 s −1 with H as target, and 2.2 × 10 −10 cm 3 s −1 with D). For collisions with molecules, the reactivity increases significantly with falling temperature, reaching the Langevin values at 15 K. These results are reported as k = α ( T /300 K) β with α = 9.5 × 10 −10 cm 3 s −1 and β = −0.15 for H 2 and α = 4.9 × 10 −10 cm 3 s −1 and β = −0.30 for D 2 .

The interaction of D(-) with H was studied experimentally and theoretically at low temperatures. The rate coefficients of associative detachment and electron transfer reactions were measured in the temperature range 10-160 K using a combination of a cryogenic 22-pole trap with a cold effusive beam of atomic hydrogen. Results from quantum-mechanical calculations are in good agreement with the experimental data. The rate coefficient obtained for electron transfer is increasing monotonically with temperature from 1 × 10(-9) cm(3) s(-1) at 10 K to 5 × 10(-9) cm(3) s(-1) at 160 K. The rate coefficient for associative detachment has a flat maximum of 3 × 10(-9) cm(3) s(-1) between 30 and 100 K.

In single nanoparticle mass spectrometry, individual charged nanoparticles (NPs) are trapped in a quadrupole ion trap and detected optically, allowing their mass, charge, and optical properties to be monitored continuously. Previous experiments of this type probed NPs that were either fluorescent or large enough to detect by light scattering. Alternatively, small NPs can be heated to temperatures where thermally excited emission is strong enough to allow detection, and this approach should provide a new tool for measurements of sublimation and surface reaction kinetics of materials at high temperatures. As an initial test, we report a study of carbon NPs in the 20-50 nm range, heated by 10.6 μm, 532 nm, or 445 nm lasers. The kinetics for sublimation and oxidation of individual carbon NPs were studied, and a model is presented for the factors that control the NP temperature, including laser heating, and cooling by sublimation, buffer gas collisions, and radiation. The estimated NP temperatures were in the 1700-2000 K range, and the NP absorption cross sections ranged from ∼0.8 to 0.2% of the geometric cross sections for 532 nm and 10.6 μm excitation, respectively. Emission spectra of single NPs and small NP ensembles show a feature in the IR that appears to be the high energy tail of the thermal (blackbody-like) emission expected from hot particles but also a discrete feature peaking around 750 nm. Both the IR tail and 750 nm peak are observed for all particles and for both IR and visible laser excitation. No significant difference was observed between graphite and amorphous carbon NPs.

Chemical models used to study the chemical composition of the gas and the ices in the interstellar medium are based on a network of chemical reactions and associated rate coefficients. These reactions and rate coefficients are partially compiled from data in the literature, when available. We present in this paper kida.uva.2014, a new updated version of the kida.uva public gas-phase network first released in 2012. In addition to a description of the many specific updates, we illustrate changes in the predicted abundances of molecules for cold dense cloud conditions as compared with the results of the previous version of our network, kida.uva.2011., Accepted for publication in ApJS on February 26th, 2015

Measurements on the energetic structure of the dissociative recombination rate coefficient in the millielectronvolt range are described for ions produced in the lowest rotational levels by collisional cooling and stored as a fast beam in the magnetic storage ring TSR (Test Storage Ring). The observed resonant structure is consistent with that found previously at the storage ring facility CRYRING in Stockholm, Sweden; theoretical predictions yield good agreement on the overall size of the rate coefficient, but do not reproduce the detailed structure. First studies on the nuclear spin symmetry influencing the lowest level populations show a small effect different from the theoretical predictions. Heating processes in the residual gas and by collisions with energetic electrons, as well as cooling owing to interaction with cold electrons, were observed in long-time storage experiments, using the low-energy dissociative recombination rate coefficient as a probe, and their consistency with the recent cold measurements is discussed.

The ion and its deuterated isotopologues H 2 D + , D 2 H + and play an important role in astrophysical and laboratory plasmas. The main challenge for understanding these ions and their interaction at low temperatures are state-specific experiments. This requires manipulation and a simple but efficient in situ characterization of their low-lying rotational states. In this contribution we report measurements of near infrared (NIR) absorption spectra. Required high sensitivity is achieved by combining liquid nitrogen cooled plasma with the technique of NIR cavity ringdown absorption spectroscopy. The measured transition frequencies are then used for exciting cold ions stored in a low-temperature 22-pole radiofrequency ion trap. Absorption of a photon by the stored ion is detected by using the laser-induced reactions technique. As a monitor reaction, the endothermic proton (or deuteron) transfer to Ar is used in our studies. Since the formed ArH + (or ArD + ) ions are detected with near unit efficiency, the stored ions can be characterized very efficiently, even if there are just a few of them.

A study of D 2 H + ions in their lowest rotational states is presented. The ions are generated in pulsed discharge in liquid N 2 cooled He/Ar/H 2 /D 2 gas mixture. Near infrared (NIR) second overtone transitions in the 6534–6536 cm −1 (1.529–1.530 μm) region are used to identify the ions and determine their degree of rotational excitation. The data were obtained using NIR cavity ringdown absorption spectroscopy (NIR-CRDS). The sensitivity obtained was typically 5 × 10 −9 cm −1 . The measured second overtone transition frequencies are in very good agreement (better than 0.02 cm −1 ) with ab initio predictions. From the Doppler broadening the kinetic temperature of ions is estimated to be (220 ± 50) K. The absolute number density of D 2 H + as a function of H 2 /D 2 mixing ratio and time is measured.

An apparatus has been developed with the aim of measuring the electronic spectra of large ions at low vibrational and rotational temperatures, as of relevance to astronomical observations. The mass-selected ions are injected into a 22-pole radio frequency trap where internal degrees of freedom are deactivated by cryogenically cooled helium gas. In these first experiments the electronic transitions are detected by a predissociation process for N2O C and para-dichlorobenzene cations (p-DCB C ). In case of N2O C the ~ A 2 S C ) ~ X 2 P3=2 transition is rotationally resolved and the population fits to Trotw25 K. The ~ B 2 B3u) ~ X 2 B2g system of

Manydeuteratedmoleculeshavebeendiscoveredininter-andcircumstellarregions.Insomecases,theobserved abundances can be explained with simple thermodynamic models; often, however, isotope enrichment is more complicated. This has been seen recently in detailed low-temperature experiments performed for the fundamental systemsH þ =H2D þ andCH þ =CHn� 1D þ .An unsolvedproblem is toexplainthelarge abundanceofC3H2andlarger hydrocarbons and their deuterated variants observed in cold, dark interstellar clouds. In this work a variabletemperature22poletrapisutilizedforcloselyscrutinizingvariousion-moleculereactionsthatmaycontributetothe formation of C3H2D þ or C3HD þ and, via dissociative recombination, C3HD. The experimental study of the promising candidate C3H þ þ HD ! C3H2D þ þ H2, which has already been excluded by theory, corroborates that this exothermic H-D exchange does not occur at all. A careful analysis of the data reveals that the 15 K rate coefficient is smaller than 4 ; 10 � 16 cm 3 s � 1 . In contrast, quite efficient routes have been found in the lowtemperature experiments, starting with C þ and proceeding via deuterated C3H þ to C3H þ and C3H þ . Formation of C3D þ in C þ þ HD collisions is 6 times faster than assumed in astrochemical models (k ¼ 9:3 ; 10 � 10 cm 3 s � 1 ). Surprisingly, direct production of C3HD þ via radiative association has also been observed (kr ¼ 6:0 ; 10 � 11 cm 3 s � 1 ).ReactionsofpartlyorfullydeuteratedC3H þ þ H2 collisionsystemarestronglydependentontemperatureand thehydrogenorthotopararatio.Inaddition,itshowsverycomplicatedisotopeeffects.Forexample,inC3H þ þ HD collisions the formation of C3HD þ (k ¼ 4:6 ;10 � 10 cm 3 s � 1 ) dominates over the H-D exchange (k ¼ 5:6 ; 10 � 11 cm 3 s � 1 ) and radiative association C3H2D þ (kr ¼ 3:2 ;10 � 11 cm 3 s � 1 ). The reactions of C3H þ þ H2 are very slow for all isotope combinations. Although several questions remain open and more low-temperature experiments are needed, it is recommended that the new values are included in astrochemical databases, since they are fundamental to the correct description of the carbon chemistry in interstellar clouds.

A new experimental set-up has been developed for studying astrochemically relevant collisions between small neutral carbon molecules C n ( n = 1–3) and stored ions. The ions are confined for seconds in a ring electrode trap (RET) the temperature of which can be varied over a wide range (presently 80–600 K). There they interact with an effusive carbon beam, which is produced via high-temperature vaporization of a carbon rod. Due to the accessible temperature range and other features of the set-up, rate coefficients can be measured, which are of importance for understanding the chemistry occurring in the outflow of stars, the formation of hydrocarbons in stellar atmospheres, and the interaction of the stored product ions with radiation. Results are reported for the interaction of stored D 3 + with hot C n . The D + transfer dominates over all other exothermic product channels for n = 1–3. The reaction rate coefficients measured for forming C n D + are almost a factor two smaller than values presently used in astrophysical models. Another important class of reactions concerns the growth of pure carbon chains via associative C m + + C n collisions. First results indicate that the rate coefficients are slower than generally assumed in models. Due to the weak signal, only rough limits can be reported. For future studies, the number density of carbon penetrating the trap must be increased. This and the planned extension of the temperature range is briefly discussed in the outlook.

Deuteration of small hydrocarbon ions CH via H-D exchange (n = 3-5) has been studied in a 22 pole ion trap at a nominal temperature of 15 K. Sequential deuteration from CH to CD is very fast if one uses pure HD as the target gas. Rate coefficients have been measured to be 1.65 × 10-9, 1.59 × 10-9, and 1.50 × 10-9 cm3 s-1. If, however, CH is relaxed in p-H2 containing traces of HD, the rate coefficient for isotope enrichment is significantly smaller, (4 ± 2) × 10-10 cm3 s-1. This important result is most probably due to symmetry selection rules influencing this reaction. The ions CH and CH are not observed, within the experimental uncertainties, to exchange H atoms for D atoms at all. Upper limits for the rate coefficients for forming CH3D+ and CH4D+ are 1 × 10-12 and 5 × 10-18 cm3 s-1, respectively. Hydrogen or deuterium abstraction in collisions of CH with HD occurs with a sum rate coefficient of 4.5 × 10-10 cm3 s-1. Surprisingly, the more exoenergetic and statistically favored product CH4D+ is formed only in 1/3 of the reactive collisions, while CH dominates with 2/3. The results are discussed on the basis of the formation of long-lived collision intermediates, open- and closed-shell ions, and barriers along the reaction path. All experimental data clearly indicate that conservation of total nuclear spin plays an important role in these low-temperature chemical reactions involving identical nuclei. Implications of this laboratory work to isotopic fractionation in astrophysical environments are discussed.

Reactions of methane cations, CH þ , with H2, HD and D2 have been studied in a variable temperature 22-pole ion trap from 9 room temperature down to 15 K. The formation of CH þ in collisions with H2 is slow at 300 K, but it becomes faster by at least one 0 order of magnitude when the temperature is lowered to 15 K. This behavior is tentatively explained with a longer complex lifetime at low temperatures. However, since tunneling is most probably not responsible for product formation, other dynamical or statistical 2 restrictions must be responsible for the negative temperature dependence. In collisions of CH þ with HD, the CH þ product ion (68% at 15 K) prevails over CH4D þ (32%). Reaction of CH þ with D2 is found to be much slower than with H2 or HD. The rate coefficient 4 for converting CH þ into CH3D þ by H–D exchange has been determined to be smaller than 10 � 12 cm 3 /s, indicating that scrambling 5 in the CH þ complex is very unlikely.

Highresolution nanoparticle mass spectrometry (NPMS) is used to study the interaction of electrons and molecules with the surface of a single, isolated particle stored in a three-dimensional quadrupole trap over weeks. IR-laser heating is employed as a fast temperature control. The kinetics of adsorption and desorption of molecules is studied for a 500-nm-diameter SiO2 particle. A C60 multilayer film has been prepared during online NPMS monitoring. Emission probabilities for secondary electrons are determined for a bare particle and a particle with a 40-nm-thick layer of C60. From the molecular desorption rates (∼fg/h) at constant temperature binding energies of multilayer 1.47-eV and submonolayer 1.53-eV C60 have been determined. Future perspectives of this new surface-science technique are discussed.

A new tandem mass spectrometer, containing a temperature-variable 22-pole ion trap, has been constructed. It is applied, as a first application, to kinetic and spectroscopic investigation of charged water clusters produced from a supersonic expansion. Using low-pressure He or H2 as buffer gas for collisional thermalization, refrigeration of the ion trap allows a good control of the cluster temperature over the range 77−350 K. It provides an accurate means of determining the dissociation energies of both protonated and deprotonated water cluster ions [H+(H2O)n and OH-(H2O)m] by measuring the dissociation rates at various temperatures along with their internal energies calculated from vibrational frequencies provided by density functional theory calculations. In this report, results of the thermochemical measurements for H+(H2O)4-10 and OH-(H2O)3-7 at well-defined cluster temperatures are presented. The feasibility of using this ion trap to acquire temperature-dependent infrared spectra of charged water clu...

For studying reactions between ions and metal atoms, a temperature variable ring electrode trap (RET) has been combined with an atomic beam source. In Section 2 , special attention is given to the calibration of the number density of the Ni atoms, injected along the axis of the trap from a metal evaporator. In Section 3 , an unexpected isotope effect is reported for the reaction O 2 + +Ni→NiO + +O. The isotope 58 Ni reacts with a rate coefficient k (670 K )=(8±2)×10 −10 cm 3 s −1 , whereas 60 Ni reacts two times slower. Formation of Ni 2 + dimers by ternary reactions with He, H 2 , and D 2 as buffer gas is very slow while, in the presence of N 2 , efficient formation of Ni 2 + has been observed via a catalytic cycle utilizing Ni(N 2 ) n + , n =1–3, as intermediate. The rate coefficient for production of Ni 2 + in collisions of NiN 2 + with Ni has been determined to be k (1060 K )=(1.7±0.7)×10 −10 cm 3 s −1 .

This report summarizes recent advances in using innovative storage devices based on inhomogeneous, time-dependent electric fields. Electrode arrangements include quadrupoles, linear octopoles, higher-order multipoles, or a series of ring electrodes. Applying suitable oscillating voltages to such structures, with frequencies ranging from GHz via MHz to a few Hz, a variety of charged particles can be confined ranging from electrons via molecular ions to nanoparticles with masses of more than 1010 u. This contribution give a short summary of the theory, especially the effective potential approximation. The features of the technique are illustrated by several examples such as trapped ion beams, buffer gas cooling of ions in a 22-pole trap, and nanopaiticle mass spectrometry in a quadrupole. Recent experimental results include growth of structures, isotope enrichment, and submonolayer gas ad- and desorption on a stored 500 nm SiO2 particle. In the summary a few hints concerning ongoing experiments are given such as laser induced reactions, combination of traps with atomic beams, state-selected perturbation of a trapped low temperature ensemble with an infrared laser, and future trends in nanoparticle research.

The recent detection of the metastable HOC+ isomer toward a wide range of interstellar environments has demonstrated abundances that cannot be explained within current chemical models. The abundance of HOC+ in these models relies heavily on the rate of the isomerization of HOC+ by H2 to the lowest energy isomer, HCO+. A variable temperature 22-pole ion-trap apparatus is employed to study the isomerization of HOC+ by H2 at 25 K. The observed rate coefficient for isomerization is (3.8 ± 0.5) × 10-10 cm3 s-1. This indicates that there is probably no temperature dependence in this reaction below 300 K, suggesting the absence of any significant energetic barrier on the lowest adiabatic H3CO+ potential surface. This result argues that in regions where abundant HOC+ is observed, the production of this isomer must be strongly favored or that there is a concomitant rapid physicochemical loss for the isomer HCO+. Some results for forming DCO+ are briefly mentioned.

The system of reactions H3++HD↔H2D++H2 has been studied in a low-temperature multipole ion trap at a nominal temperature of 10 K . The rate coefficient k1 for the forward reaction has been found to be 3.5×10 −10 cm 3 s −1 at 10 K , a value significantly smaller than the currently accepted value of 1.5×10 −9 cm 3 s −1 . The rate coefficient k−1 for the backward reaction has been found to be much higher than the value derived from the equilibrium coefficient of ∼10 −18 cm 3 s 1 . For normal-hydrogen, a value of k −1 =4.9×10 −11 cm 3 s −1 has been deduced while for almost pure para-hydrogen (purity 99±1%) the value drops to k −1 =7.3×10 −13 cm 3 s −1 . The results are discussed on the basis of the well-known energy levels of the involved molecules and the potential energy surface of the H4D+ intermediate. Zero-point energy plays a key role; however, there are additional complications due to the formation of rotationally excited H2D+, if even traces of ortho-H2 (o-H2) are present. The consequences of the results for the chemistry of cold clouds are illustrated using an evolutive gas-phase chemical model. There is strong evidence, that the new results significantly reduce the efficiency of isotope fractionation via gas-phase reactions. The experimental results also indicate the need for state-to-state rate coefficients in order to correctly simulate the o-H2 induced non-equilibrium conditions prevailing both in the low temperature ion trap and in interstellar clouds.

In order to understand quantitatively the enrichment of deuterated molecules observed in non-equilibrium environments such as interstellar clouds, one has to know in detail many state-to-state cross sections. For detailed models, one has to treat both chemical processes such as formation and destruction of the isotopomers and physical processes such as collisional excitation and radiative transitions of the relevant molecular states. This contribution gives a short summary of experimental techniques in the field of low-energy gas-phase collisions such as flow techniques, traps, and beam methods with special emphasis on those methods which can be used to study the dynamics of H–D scrambling. The situation is illustrated with the astrophysically important H++H2 collision system in several isotopic variants. This fundamental system is well understood, and most experimental results are in good accordance with predictions from a dynamically biased statistical model. Less well understood are the different isotopic combinations of H3+ colliding with H2. The H3++HD variant is discussed in a separate paper in this special issue (H3++HD↔H2D++H2:low-temperature laboratory measurements and interstellar implications, Planet. Space Sci., this volume) while, in this contribution, the situation is illustrated with rate coefficients for isotopic exchange in D3++H2 collisions. Other examples include the study of deuteration of hydrocarbon ions in a trap at a nominal temperature of 10 K . In particular, the rate coefficients for sequential deuteration of C2H2+ in collisions with HD have been measured to be 7.5×10 −10 cm 3 s −1 and 7.0×10 −10 cm 3 s −1 . Another example refers to the reactions that occur when CH3+ is stored in para-hydrogen (p-H2) or normal-hydrogen (n-H2) containing the natural abundance of HD. At the low densities used, H–D exchange competes only with radiative association. Some hints to the next generation of experiments are given. One of the aims is to study the role of H atoms and D atoms in low-temperature gas-phase chemistry. Another aim is to combine laser and trapping techniques in order to get both state specific rate coefficients and spectroscopic information.

A sensitive experimental method for ion spectroscopy and state specific reaction dynamics is described, briefly called laser induced reactions (LIR). The technique is based on (i) trapping ions over a long time in a cold 22-pole rf ion trap followed by mass spectrometric detection, (ii) providing a suitable low density gas environment for collisions, (iii) modifying the low temperature chemical kinetics using selective excitation via a tunable radiation source. In this paper, the H-atom transfer reaction C2H2+ (v3=1,J)+H2→C2H3++H, is used to monitor the infrared excitation of acetylene ions. Rotationally resolved spectra are presented for the antisymmetric C–H stretching vibration. For recording a spectrum, it is sufficient to fill the trap with a few thousand parent ions. Differences with respect to conventional IR spectroscopy are discussed, especially the processes which influence the LIR signal. From the measured intensities and their dependence on parameters such as storage time, laser fluence and ta...

Ternary rate coefficients for the title reaction have been measured using a rf 22-pole ion trap in the temperature range 65–300 K. For the formation of dimers, the ternary rate coefficient, k3, follows the power law 2.3×10−28 cm6/s×(300 K/T)m, with m=1.6±0.2 over the full temperature range of the experiment. This result is in good agreement with previous measurements at higher temperatures and also with theoretical predictions. The formation of trimers is represented by k3=1.3×10−29 cm6/s×(300 K/T)m, with m=3.2±0.3. Including into the evaluation also previously published data measured at higher temperatures, leads to an even steeper temperature dependence. These findings are tentatively explained by the fact that (CO)2+ is a nonlinear molecule with low lying vibrational modes. The formation of tetramers, (CO)4+, is very slow and shows a positive or near zero temperature dependence. This behavior is explained by the presence of two isomers. In order to pass this bottleneck and to reach (CO)5+ and larger cl...

Two-color IR-IR isomer selective predissociation spectra of helium-tagged C6H6(2+) are presented. The dications are generated via electron bombardment of either benzene or 1,3-cyclohexadiene. After mass selection they are injected into a 2.6 K cold ion trap where the presence of a dense He buffer gas not only cools them but also leads to He attachment. The ion ensemble is exposed to one or two intense IR pulses from optical parametric oscillators (OPOs) (1200-3100 cm(-1)) before it is extracted, mass analyzed, and detected. On the basis of a comparison with theoretical predictions, the resulting spectral features allow us to separate and assign different isomers of C6H6(2+) dications. Compression of the ion cloud very close to the axis of the linear quadrupole trap and coaxial superposition of well-collimated laser beams results in the fragmentation of almost all helium complexes at specific wavelengths. This unique feature enables us to record fluence-dependent attenuation curves for individual absorption bands and thus determine not only absorption cross sections but also the composition of the ion mixture.