Infrared Spectroscopy of Neutral Clusters Based on a Vacuum Ultraviolet Free Electron Laseri

Spectroscopic characterization of clusters is crucial to understanding the structures and reaction mechanisms at the microscopic level,but it has been proven to be a grand challenge for neutral clusters because the absence of a charge makes it di伍cult for the size selection and detection.Infrared(IR)spectroscopy based on threshold photoionization using a tunable vacuum ultraviolet free electron laser(VUV-FEL)has recently been developed in the lab.The IR-VUV depletion and IR+VUV enhancement spectroscopic techniques open new avenues for size-selected IR spectroscopies of a large variety of neutral clusters without confinement(i.e.,an ultraviolet chromophore,a messenger tag,or a host matrix).The spectroscopic principles have been demonstrated by investigations of some neutral water clusters and some metal carbonyls.Here,the spectroscopic principles and their applications for neutral clusters are reviewed.

Vacuum Ultraviolet Free-Electron Laser Photoionization Mass Spectrometry of Alpha-pinene Ozonolysis

α-pinene is the most abundant monoterpene that represents an important family of volatile organic compounds.Molecular identification of key transient compounds during theα-pinene ozonolysis has been proven to be a challenging experimental target because of a large number of intermediates and products involved.Here we exploit the recently developed hybrid instruments that integrate aerosol mass spectrometry with a vacuum ultraviolet free-electron laser to study theα-pinene ozonolysis.The experiments ofα-pinene ozonolysis are performed in an indoor smog chamber,with reactor having a volume of 2 m^(3) which is made of fluorinated ethylene propylene film.Distinct mass spectral peaks provide direct experimental signatures of previously unseen compounds produced from the reaction ofα-pinene with O_(3).With the aid of quantum chemical calculations,plausible mechanisms for the formation of these new compounds are proposed.These findings provide crucial information on fundamental understanding of the initial steps ofα-pinene oxidation and the subsequent processes of new particle formation.

佛克脱线型HeⅠ1 083．0nm共振线逃逸几率的计算

基于精确佛克脱线型的表达式,对于平板几何体和柱形几何体,计算了组成HeⅠ1083.0nm共振线1083.034nm,1083.025nm和1082.908nm三条线的逃逸几率,同时计算了吸收原子在基态的原子数密度,分析了逃逸因子对谱线中心光学深度的影响。理论分析和实验结果相一致,此计算对于探测太阳或其它天体中氦的浓度具有一定参考意义。

Crossed Molecular Beam Study of the F+D2（v=1, j=0） Reaction

The reaction dynamics of the fluorine atom with vibrationally excited D2(v=1, v=0) was investigated using the crossed beam method. The scheme of stimulated Raman pumping was employed for preparation of vibrationally excited D2 molecules. Contribution from the reaction of spin-orbit excited F*(2P1/2) with vibrationally excited D2 was not found. Reaction of spin-orbit ground F(2P3/2) with vibrationally excited D2 was measured and DF products populated in v'=2, 3, 4, 5 were observed. Compared with the vibrationally ground reaction, DF products from the vibrationally excited reaction of F(2P3/2)+D2(v=1, j=0) are rotationally “hotter”. Differential cross sections at four collision energies, ranging from 0.32 kcal/mol to 2.62 kcal/mol, were obtained. Backward scattering dominates for DF products in all vibrational levels at the lowest collision energy of 0.32 kcal/mol. As the collision energy increases, angular distribution of DF products gradually shifts from backward to sideway. The collision-energy dependence of differential cross section of DF(v’=5) at forward direction was also measured. Forward-scattered signal of DF(v'=5) appears at the collision energy of 1.0 kcal/mol, and becomes dominated at 2.62 kcal/mol.

High Resolution Crossed Molecular Beams Study of the H+HD→H2+D Reaction

The H+H2 reaction is the simplest chemical reaction system and has long been the prototype model in the study of reaction dynamics. Here we report a high resolution experimental investigation of the state-to-state reaction dynamics in the H+HD→H2+D reaction by using the crossed molecular beams method and velocity map ion imaging technique at the collision energy of 1.17 eV. D atom products in this reaction were probed by the near threshold 1+1'(vacuum ultraviolet+ultraviolet) laser ionization scheme. The ion image with both high angular and energy resolution were acquired. State-to-state differential cross sections was accurately derived. Fast forward scattering oscillations, relating with interference effects in the scattering process, were clearly observed for H2 products at H2(v'=0,j'=1) and H2(v'=0,j'=3) rovibrational levels. This study further demonstrates the importance of measuring high-resolution differential cross sections in the study of state-to-state reaction dynamics in the gas phase.

Differential Cross Sections of F+HD→DF+H Reaction at Collision Energies from 3.03 meV to 17.97 meV

The prototypical reaction of F+HD→DF+H was investigated at collision energies from 3.03 meV to 17.97 meV using a crossed molecular beam apparatus with multichannel Rydberg tagging time-of-flight detection.Significant contributions from both the BornOppenheimer(BO)forbidden reaction F^(*)(^(2)P_(1/2))+HD→DF+H and the BO-allowed reaction F(^(2)P_(3/2))+HD→DF+H were observed.In the backward scattering direction,the contribution from the BO-forbidden reaction F^(*)(^(2)P_(1/2))+HD was found to be considerably greater than the BO-allowed reaction F(^(2)P_(3/2))+HD,indicating the non-adiabatic effects play an important role in the dynamics of the title reaction at low collision energies.Collision-energy dependence of differential cross sections(DCSs)in the backward scattering direction was found to be monotonously decreased as the collision energy decreases,which does not support the existence of resonance states in this energy range.DCSs of both BO-allowed and BO-forbidden reactions were measured at seven collision energies from 3.03 meV to 17.97 meV.It is quite unexpected that the angular distribution gradually shifts from backward to sideway as the collision energy decreases from 17.97 meV to 3.03 meV,suggesting some unknown mechanisms may exist at low collision energies.

Effects of Single Quantum Rotational Excitation on Reaction of F+D_(2) at Collision Energies between 44 and 164 cm^(-1)

There is no general picture to describe the influences of reagent rotational excitation on the reaction,which proceeds via the tunnelling mechanism at collision energies far below the reaction barrier.Here we report a crossed beam study on the prototypical reaction of F+D_(2)(v=0,j=0,1)→DF(v′)+D at collision energies between 44 and 164 cm^(-1)with the scheme of multichannel D-atom Rydberg tagging time-of-flight detection.Vibrational state resolved differential cross sections are obtained at v′=2,3,4 levels.The effects of reagent rotational excitation were investigated at an equivalent amount of total energy by precise tuning of translational energies.Compared with translation,the rotation of D_(2) is found to be more efficient to promote the title reaction.Profound differences introduced by rotation of D_(2) are also observed on the angular distribution and quantum state distribution of DF products.We hope the present work could provide an example for understanding the effects of reagent rotational excitation on the chemical reaction at energies that are much lower than the reaction barrier.

Vacuum Ultraviolet Photodissociation Dynamics of OCS via the F Rydberg State:theS(^(3)P_(J=2,1,0))Product Channels

Vacuum ultraviolet(VUV)photodissociation dynamics of carbonyl sulfide was investigated experimentally by using a tunable photolysis light source and the timesliced velocity map ion imaging technique.Ion images of S(^(3)P_(J=2,1,0))dissociation products were measured at five photolysis wavelengths from 133.26 nm to 139.96 nm,corresponding to the F Rydberg state of OCS.Two dissociation channels:S(^(3)P_(J))+CO(X^(1)Σ+)and S(^(3)P_(J))+CO(A^(3)Π)were observed with the former being dominant.The vibrational states of CO co-products were partially resolved in the ion images.The product total kinetic energy releases,anisotropy parameters(β),and the branching ratios of high-lying CO vibrational states were determined for the S(^(3)P_(J))+CO(X^(1)Σ^(+))channel.We found that the anisotropy parameters suddenly changed from negative to positive when OCS was excited to the higher vibrational levels of the F state.Furthermore,the anisotropy parameters for S(^(3)P_(J))products of J=2,1,0 were even different.These anomalous phenomena may result from the simultaneous existence of both parallel and perpendicular dissociation mechanisms,suggesting the involvement of other electronic states with different symmetry in the initially-excited energy region.This work provides a further understanding of the nonadiabatic couplings in the VUV photodissociation process of OCS.

Photodissociation Dynamics of CS_(2) Near 204 nm: The S(^(3)PJ)+CS(X^(1)∑^(+)) Channels

We study the phot odissociation dynamics of CS2 in the ultraviolet region using the time-sliced velocity map ion imaging technique.The S(3 Pj)+CS(X1E+)product channels were observed and identified at four wavelengths of 201.36,203.10,204.85 and 206.61 nm.In the measured images of S(3Pj=2,1,0),the vibrational states of the CS(X1E+)co-products were partially resolved and the vibrational state distributions were determined.Moreover,the product total kinetic energy releases and the anisotropic parameters were derived.The relatively small anisotropic parameter values indicate that the S(3Pj=2丄0)+CS(X1E+)channels are very likely formed via the indirect predissociation process of CS2.The study of the S(3Pj=2,1;0)+CS(X1E+)channels,which come from the spin-orbit coupling dissociation process of CS2,shows that nonadiabatic process plays a role in the ultraviolet photodissociation of CS2.

Wavelength Dependent Photodissociation of OCS via F 3^1ПRydberg State:CO(X1+)+S(1D2)Product Channel

The vacuum ultraviolet photodissociation of OCS via the F 3^1ΠRydberg states was investigated in the range of 134-140 nm by means of the time-sliced velocity map ion imaging technique.The images of S(^1D2)products from the CO(X^1Σ^+)+S(^1D2)dissociation channel were acquired at five photolysis wavelengths,corresponding to a series of symmetric stretching vibrational excitations in OCS(F 3^1Π,v1=0-4).The total translational energy distributions,vibrational populations and angular distributions of CO(X^1Σ^+,v)coproducts were derived.The analysis of experimental results suggests that the excited OCS molecules dissociate to CO(X^1Σ^+)and S(^1D2)products via non-adiabatic couplings between the upper F 3^1Πstates and the lower-lying states both in the C∞v and Cs symmetry.Furthermore,strong wavelength dependent behavior has been observed:the greatly distinct vibrational populations and angular distributions of CO(X^1Σ^+,v)products from the lower(v1=0-2)and higher(v1=3,4)vibrational states of the excited OCS(F 3^1Π,v1)demonstrate that very different mechanisms are involved in the dissociation processes.This study provides evidence for the possible contribution of vibronic coupling and the crucial role of vibronic coupling on the vacuum ultraviolet photodissociation dynamics.

Crossed Beam Experiment on the Validity of Born-Oppenheimer Approximation in Cl(2P)+D2→DCl+D Reaction

The reaction of chlorine atom Cl(2P)(Cl(2P3/2)and Cl^*(2P1/2))with D2 was investigated at collision energy from 4.5 kcal/mol to 6.5 kcal/mol with a high-resolution crossed molecular beam apparatus using the technique of D-atom Rydberg tagging detection.The contribution from the spin-orbit excited reaction Cl^*(2P1/2)+D2,which is prohibited by Born-Oppenheimer(BO)approximation,was observed.Collision-energy dependence of differential cross sections(DCSs)near the backward scattering direction was measured.The BOforbidden reaction Cl^*+D2 was found to be dominant at lower collision energy.As collision energy increases,reactivity of BO-allowed reaction Cl+D2 increases much faster than that of BO-forbidden reaction and becomes dominant at higher collision energy.Our experiment indicates that the additional energy of spin-orbit excitation in Cl^*facilitates BO-forbidden reaction to pass through the barrier at lower collision energy,while BO approximation is still valid at collision energy near and above the reaction barrier.This tendency of reactivity of Cl/Cl^*+D2 is similar to the isotopic reaction of Cl/Cl^*+H2.

Photodissociation Dynamics of OCS Near 128 nm:S(3PJ=2,1,0),S(1D2)and S(1S0)Channels

Here we report the study of the photodissociation dynamics of carbonyl sulfide in the vacuum ultraviolet region using the time-sliced velocity map ion imaging technique.Images of S(^3PJ=2,1,0),S(^1D2)and S(^1S0)products were measured at four photolysis wave-lengths of 129.32,128.14,126.99,and 126.08 nm,respectively.Four main dissociation channels:S(^3PJ=2,1,0)+CO(X^1Σ^+),S(^3PJ=2,1,0)+CO(A^3π),S(^1D2)+CO(X^1Σ^+)and S(^1S0)+CO(X^1Σ^+)channels,have been clearly observed and identified.Vibrational states of the CO co-products were partially resolved in the experimental images.From these images,the product total kinetic energy releases,the branching ratios and angular distributions of products have been derived.While the S(^3PJ=2,1,0)+CO(A^3π)product channel is formed through the adiabatic dissociation process after the excitation to the(3^1Σ^+)excited state,the results suggest that strong nonadiabatic coupling plays an important role in the formation of other three channels.

Coverage Dependent Dissociative Adsorption of HCl on Au(111)

Dissociative adsorption of HCl on Au(111)has become one of unsolved puzzles in surface chemistry.Despite tremendous efforts in the past years,varioustheoretical models still greatly overestimate the zero-coverage initial sticking probabilities(So).To find the origin of the large experiment-theory discrepancy,we have revisited the dissociative adsorption of HCl on Au(111)with a newly designed molecular beam-surface apparatus.The zero-coverage So derived from Cl-coverage measurements with varying HCl doses agree well with previous ones.However,we notice a sharp change of the coverage/dose slope with the HCl dosage at the low coverage regime,which may result in some uncertainties to the fitted So value.This seems consistent with a coverage-dependence of the dissociation barrier predicted by density functional theory at low Cl-coverages.Our results reveal the potential inconsistency of utilizing simulations with finite coverage to compare against experimental data with zero coverage in this system,and provide guidance for improving both experiment and theory in this regard.