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The article focuses on a discussion about scattering in extreme environments, specifically addressing the impact of molecular orientation on nuclear spin coupling measurements. Topics include the effect of relative orientation on measurement outcomes, the role of angular momentum in molecular collisions, and the absence of significant orientation effects in the studied system.

The article focuses on the discussion of scattering phenomena involving larger molecules, specifically addressing questions related to the phenylethynyl radical in the interstellar medium. Topics include the challenges in detecting the phenylethynyl radical due to its complex spectrum, potential formation mechanisms in space, and the role of experimental and computational methods in predicting its presence.

The article focuses on a general discussion of scattering phenomena at condensed-phase surfaces, particularly in relation to the challenges and techniques involved in measuring angular distributions of evaporating atoms. Topics include the difficulties of measuring angular distributions from cylindrical jets versus flat jets, the prediction of helium atom evaporation patterns, and the observation of surfactant segregation in microjets.

The article focuses on a general discussion about the scattering of larger molecules, particularly examining the differences in potential energy surfaces (PES) between HDO, H2O, and D2O. Topics include the reason for stronger back scattering in HDO compared to H2O and D2O, the role of symmetry and potential terms in scattering phenomena, and specific insights into the n10 term affecting back-scattering.

Single fluorophores and single-pair fluorescence resonance energy transfer are studied with a new confocal fluorescence microscope that allows, the wavelength and emission time of each detected photon to be simultaneously measured with single molecule sensitivity. The initial results from a simultaneous time- and wavelength-resolved single molecule confocal microscope system are presented.

An investigation is conducted on the dissociation dynamics of the ArNO van der Waals molecule near 225 nm. The appearance threshold for products from dissociation of ArNO is measured to produce NO(A,N=O) as 44291 plus minus 2 cm.

The detection and real-time measurement of circular dichroism (CD) in liquid samples was demonstrated using a new pulsed four-wave mixing technique. The technique uses interference from two laser beams with polarizations controlled and modulated using a photoelastic modulator. An internal heterodyne process was used and interference from coherent thermal gratings was found to enhance a weak circular dichroism signal.

We use a simple, forward-geometry degenerate four-wave mixing setup to monitor weak absorptions of various gas-phase molecules with a tunable excimer laser. With this technique, state-selective detection of H2, CO, H2O, and O2 is demonstrated. The dependence of the forward-geometry degenerate four-wave mixing signals on various experimental parameters (e.g., pressure, laser power) is determined. Several mechanisms for the generation of the observed signals are discussed, including diffraction from density gratings or thermal gratings and two-photon degenerate four-wave mixing.

The dissociation of aligned, electronically excited H2 (E,F 1 Σ g + ), followed by the ionisation of the produced H atom, is analysed via the velocity mapped imaging technique. The dissociation and ionisation processes are accomplished, respectively, by a two- and a one-photon absorption from a single 532-nm laser pulse, while the alignment is induced by a separate 1064-nm laser pulse. The velocity of the produced H+ photofragments shows a weak perpendicular alignment at low alignment laser field values, evolving to strongly parallel for larger fields. We modelled this alignment behaviour with a simple two-state model involving the Stark mixing of the initially-prepared J = 0 with the J = 2 rotational state. This model is able to reproduce all of the observed angular distribution and permits us to extract from the fit the polarisability anisotropy of H2 (E,F) electronic state. We determine this value to be (3.7 ± 1.2) × 103 a.u. As this value is extremely large in comparison to what one would expect from the pure H2 (E,F) electronic state, we hypothesise that this value comes from the 1064-nm laser beam mixing nearby electronic states with the initially laser prepared (E,F) state generating a mixed state (EF**) with an extremely large polarisability anisotropy. [ABSTRACT FROM AUTHOR]

We have demonstrated a spectroscopic technique for simultaneously obtaining broad spectral bandwidth and high frequency resolution absorption measurements, with 5 μs temporal resolution, continuously for tens of microseconds in an apparatus with no active stabilization. The technique utilizes two passive air-gap etalons to imprint two frequency comb patterns onto a single pulsed light source. The air-gap etalons also serve as cavity ringdown cells increasing the sensitivity of the absorption spectroscopy by increasing the interrogation path length. Here, we demonstrate the operation of the spectrometer utilizing a ∼0.15 cm-1 bandwidth pulsed dye laser and two nearly identical 300 MHz free-spectral range confocal air-gap etalons each with a finesse of ∼1 × 105, to investigate the (1,1,3) overtone of water and the R(7) transition of the O2 b1Σg+←X3Σg- (2,0) band with high spectral resolution. [ABSTRACT FROM AUTHOR]

There is great interest in the production of cold molecules, at temperatures below 1 K, and ultracold molecules, at temperatures below 1 mK. Such molecules have potential applications in areas ranging from precision measurement to quantum information storage and processing, and quantum gases of ultracold polar molecules are expected to exhibit novel quantum phases. In addition, cold molecules open up a new domain for collision physics, dominated by long-range forces and scattering resonances. There have been major recent advances both in cooling molecules from room temperature and in forming molecules in ultracold atomic gases. As these techniques mature, and cold and ultracold samples are more accessible, collision studies at previously unavailable energies will be possible. This spotlight article will highlight some of the background and motivation for studying collisions at low energies and will direct readers to recent articles on the recent experimental advancements. [ABSTRACT FROM AUTHOR]

The origin of the resonant structures in the spectrum of the predissociative part of the A state in the NO–Ar van der Waals cluster has been investigated. We have employed direct excitation to the predissociative part of the NO–Ar A state followed by rotational state selective ionization of the NO fragment. Velocity map imaging of the NO ion yields the recoil energy of the rotational state-selected fragment. A substantial contribution of rotational hotbands to the resonant structures is observed. Our data indicate that a centrifugal barrier as the origin of these resonances can be ruled out. We hypothesize that after the NO–Ar cluster is excited to the A state sufficient mixing within the rotating cluster takes place as it changes geometry from being T shaped in the NO(X)–Ar state to linear in the NO(A)–Ar state. This mixing allows the low energy and high angular momentum (J≈4.5) tumbling motion of the initially populated hotbands in the ground state NO(X)–Ar complex to be converted into NO(A,n=2) spinning rotation in the A state of the complex. The electronically excited spinning complex falls apart adiabatically producing rotationally excited NO(A,n=2) at the energetic threshold. This interpretation indicates that the resonances can be attributed to some type of vibrational Feshbach resonance. The appearance energy for the formation of NO(A,n=0)+Ar is found to be 44294.3±1.4 cm-1. [ABSTRACT FROM AUTHOR]

We have determined limits on the cross section for both electronically nonadiabatic excitation and quenching in the Cl(2Pj)+D2 system. Our experiment incorporates crossed-molecular-beam scattering with state-selective Cl(2P1/2,3/2) detection and velocity-mapped ion imaging. By colliding atomic chlorine with D2, we address the propensity for collisions that result in a change of the spin-orbit level of atomic chlorine either through electronically nonadiabatic spin-orbit excitation Cl(2P3/2)+D2→Cl*(2P1/2)+D2 or through electronically nonadiabatic spin-orbit quenching Cl*(2P1/2)+D2→Cl(2P3/2)+D2. In the first part of this report, we estimate an upper limit for the electronically nonadiabatic spin-orbit excitation cross section at a collision energy of 5.3 kcal/mol, which lies above the energy of the reaction barrier (4.9 kcal/mol). Our analysis and simulation of the experimental data determine an upper limit for the excitation cross section as σNA≤0.012 Å2. In the second part of this paper we investigate the propensity for electronically nonadiabatic spin-orbit quenching of Cl* following a collision with D2 or He. We perform these experiments at collision energies above and below the energy of the reaction barrier. By comparing the amount of scattered Cl* in our images to the amount of Cl* lost from the atomic beam we obtain the maximum cross section for electronically nonadiabatic quenching as σNA≤15-15+44 Å2 for a collision energy of 7.6 kcal/mol. Our experiments show the probability for electronically nonadiabatic quenching in Cl*+D2 to be indistinguishable to that for the kinematically identical system of Cl*+He. [ABSTRACT FROM AUTHOR]

Rotational state-resolved differential cross sections (DCS) for spin-multiplet-changing collisions of NO(X[SUP2]Π[SUB1/2]→X[SUP2[Π[SUB3/2]) with Ar are presented and compared to results from previous experimental and theoretical studies performed on the NO/Ar system. A crossed molecular beam apparatus coupled with velocity-mapped ion imaging was used to measure complete (&theta=0°-180°) DCS for scattering of NO(X[SUP2]Π1/2],j=0.5) into NO(X[SUP2]Π3/2,j′) rotational states ranging from j′ = 1.5 to j′ = 12.5. Scattered products were detected by state-selective ionization using (1+1′) resonance-enhanced multiphoton ionization via the A[SUP2]Σ[SUP+] state. State-to-state DCS were extracted in the center-of-mass frame of reference for energy transfer at a center-of-mass collision energy of ∼530 cm[SUP-1]. Studies performed using horizontally and vertically polarized excitation laser beams yielded DCS which were remarkably similar, indicating that state-to-state scattering for this system is insensitive to probe beam polarization. Experimentally determined angular scattering distributions show primarily forward scattering for low-energy rotational states (j≤7.5), with side- and back-scattering increasing with product angular momentum. The scattering results are compared and contrasted to results from earlier experimental investigations and to theoretical results from quantum close-coupling calculations based on ab initio coupled cluster CCSD(T) potential energy surfaces. [ABSTRACT FROM AUTHOR]

A general numerical method is given to extract angular correlations from photodissociation experiments with ion imaging detection. The angular correlations among the transition dipole moment of the parent molecule, μ, the photoproduct recoil velocity, v, and its angular momentum, j, are parametrized analytically using the semiclassical bipolar moment scheme due to Dixon. The method is a forward-convolution scheme which allows quantitative extraction of all measurable bipolar moments and can be applied in experiments with both linearly and circularly polarized probe light. It avoids the cylindrical symmetry limitations of the inverse Abel transform method, traditionally used for extracting photoproduct recoil anisotropy and speed distribution from imaging data. The method presented here also takes into account the possibility of multiple photodissociation channels. The features of the method are illustrated in a two-color 1+1′ REMPI-ion imaging study of the NO photoproduct trajectories resulting from the 650 nm photodissociation of 2-chloro-2-nitrosopropane (CNP). A comparison between experimental and synthetic images is presented for selected experimental geometries. The experimental images for CNP and the results from their fit confirm earlier TOF studies showing that the recoil speed distribution is bimodal with the low and high speed components having average values of approximately 500 and 910 m/s. These components have been previously assigned to dissociation from the S[sub 0] and T[sub 1] electronic states of the parent molecule, respectively. The experimental results from the current study also confirm that for the high-speed component the product NO velocity vector, v, is preferentially perpendicular to its angular momentum, j [β[sub 0][sup 0](22)=-0.21], and that there is no significant correlation between v and the transition dipole moment μ of the CNP molecule [β[sub 0][sup 2](20)=-0.02]. © 2001 American Institute... [ABSTRACT FROM AUTHOR]

Measures the state-specific angular distributions of nitrogen produced in the ultraviolet photodissociation of nitric oxide using photofragment imaging. Ionization of individual rotational states; Transitions in the perturbed region of nitrogen; Potential energy surfaces and dynamics involved in the photodissociation process.

The results of experiments investigating the charge-transfer state photofragments of a van der Waals cluster after excitation through a strong absorption band in the ultraviolet is reported. The data suggest that the CI2 - cyclohexane cluster has an axial-like structure following absorption of a photon.

Collisions of HCL (j = 0) with rare gas and molecular colliders are studied using velocity-mapped ion imaging. It is implied that collision needs to have a smaller impact parameter than would be required for an atomic collision in order to generate HCL in a highly rotationally excited quantum state following collision with a diatomic or polyatomic collider.

Results are presented from femtosecond time-resolved coherent Raman experiments in which we excite and monitor vibrational coherence in gas-phase samples of benzene and 1,3,5-hexatriene. Different physical mechanisms for coherence decay are seen in these two molecules. In benzene, where the Raman polarizability is largely isotropic, the Q branch of the vibrational Raman spectrum is the primary feature excited. Molecules in different rotational states have different Q-branch transition frequencies due to vibration–rotation interaction. Thus, the macroscopic polarization that is observed in these experiments decays because it has many frequency components from molecules in different rotational states, and these frequency components go out of phase with each other. In 1,3,5-hexatriene, the Raman excitation produces molecules in a coherent superposition of rotational states, through (O, P, R, and S branch) transitions that are strong due to the large anisotropy of the Raman polarizability. The coherent superposition of rotational states corresponds to initially spatially oriented, vibrationally excited, molecules that are freely rotating. The rotation of molecules away from the initial orientation is primarily responsible for the coherence decay in this case. These experiments produce large (∼10% efficiency) Raman shifted signals with modest excitation pulse energies (10 μJ) demonstrating the feasibility of this approach for a variety of gas phase studies. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

In this paper we describe a method, based on the laser-induced grating technique, for studying the spectroscopy of vibrational overtone-excited gas-phase water. Two phase-coherent visible laser beams whose frequencies are in the range of the third overtone of the OH stretch in water are crossed in the gas-phase sample. As the wavelength of these excitation beams is scanned through individual rovibrational OH overtone transitions, vibrational energy is deposited into the water in a spatially sinusoidal pattern. A fixed-frequency 266 nm probe laser beam is diffracted from the resultant transmission diffraction grating in water. We show that under collision-free conditions, probe laser diffraction is observed from the initially excited grating, which is a necessary condition for using this technique to study the absorption spectroscopy of the vibrationally excited molecules. Under multiple collision conditions, a probe laser wavelength-independent refractive index grating is formed within the bulk sample. In addition, we observe temporal oscillations in the grating diffraction efficiency arising from excitation of standing acoustic waves. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A new excited-state spectroscopic method is reported.It is a two-color laser-induced grating tecnique for detecting optical transitions of rovibronically excited molecules in the gas phase. (AIP) [ABSTRACT FROM AUTHOR]

We investigate the rotational energy transfer of H35Cl (v=1) in collisions with thermal HCl at 298 K. Rotationally state-selective excitation of v=1 over J=1–6 is achieved by stimulated Raman pumping, and the rotational relaxation from the initially pumped level is monitored via 2+1 resonantly enhanced multiphoton ionization (REMPI) through the E 1Σ+–X 1Σ+0–1 band. The ions are detected in a time-of-flight mass spectrometer in order to ensure that only relaxation of H35Cl is observed. We present empirical correction factors for determining relative rotational populations from the REMPI spectral line intensities and extract the rate constants for rotational energy transfer from the time-dependent populations using numerical techniques. The excellent sensitivity of the REMPI technique makes it possible to monitor the relaxation on very short collisional time scales (<0.1 hard sphere collision) and thereby enables us to determine the rate constants for both single quantum and multiquanta rotational transitions directly, without the use of a simplifying model for the matrix of rate constants. The measured rate constants illustrate the importance of multiquanta transitions for HCl; such transitions account for ∼30% of the total population loss from a given rotational level. We compare our rate constants to those obtained in previous measurements on HCl (v=1) and HF (v=2). We find that the rate constants are not adequately described by models based solely on the rotational energy defect of the vibrationally excited molecule. [ABSTRACT FROM AUTHOR]

We have examined the photofragmentation HNCO→NH(a 1Δ)+CO using radiation at wavelengths shorter than 230 nm. Nascent NH(a 1Δ) shows relatively little rotational excitation, accounting for less than 12% of the energy in excess of the dissociation energy. The rotational state distributions evidence less population in high rotational states than predicted by statistical theories but more than expected on the basis of a simple impulsive dissociation. A semiclassical impulsive model that describes photoproduct rotation as developing during fragmentation successfully describes the rotational state distributions of NH(a 1Δ) produced by HNCO photodissociation over a wide range of wavelengths. The success of this model in describing the NH rotational state distributions and previously measured CO rotational state distributions suggests that the excited state potential energy surface may be repulsive with minima in HNC and NCO bond angles each near 120°. [ABSTRACT FROM AUTHOR]

A master-equation formalism is applied to the problem of overtone-induced isomerization of CH3NC to CH3CN. The results are compared to the experiments of Reddy and Berry, who measured the yield of isomerization as a function of pressure after excitation to the fourth and fifth overtones of the CH stretching mode. The master-equation model predicts the yield and the curvature in the yield-1 vs pressure plots observed in the experiments. For the lower overtone (5←0) the results are consistent with a simple strong-collider model. However, even under strong-collider conditions the yield is very sensitive to the parameters in the master equation. For the upper overtone (6←0) the data do not fit a strong collider model and multistep deactivation dominates. We are able to determine from the data the average energy transferred in a collision by assuming a particular form for the energy-transfer function. In addition, the effect of changing the shape of the energy-transfer function is investigated. [ABSTRACT FROM AUTHOR]

We have used tunable, pulsed ultraviolet laser radiation to prepare H2 and HD in selected rovibronic levels of the E,F 1Σ+g electronic state. This state is accessible from the ground state, X 1Σ+g , by means of two-photon excitation. Subsequent absorption of near infrared laser radiation by molecules prepared in the E,F 1Σ+g state produces ions that are detected by a time-of-flight mass spectrometer. Radiative lifetimes of individual rovibronic levels are determined by measuring the ion signal as a function of the time delay between the ultraviolet and near-infrared laser pulses. A decreasing exponential function is fit to the ion signal associated with a particular rovibrational level to obtain its liftime. Measured lifetimes are in good agreement with previously reported theoretical lifetimes and the single previous measurement. [ABSTRACT FROM AUTHOR]

We report state-to-state rotational energy transfer rates for HD excited to the first excited vibrational level of the ground electronic state. Stimulated Raman scattering is used to produce the rotationally selected, vibrationally excited HD. Subsequent collisional energy transfer from the prepared state, upon collision with a thermal distribution of HD, is monitored by multiphoton ionization through the E,F electronic state. The data are analyzed by solving the rate equations coupling the lowest six rotational states of the first excited vibrational level. In this manner, both the absolute rate constants and the shape of the energy transfer probability density function are determined. The best fit of the data to trial probability density functions indicates that the HD–HD collisions preserve the magnetic sublevel, mj. The total rotational energy transfer rate out of a particular rotational level is compared to high resolution Raman linewidth measurements in order to determine the degree to which the rotational energy transfer rate contributes to the linewidth. [ABSTRACT FROM AUTHOR]