Printed in the United Kingdom Laser Stimulation and Observation of Simple Gas Phase Radical Reactions

Experiments on the effect of Selective vibrational, translational and orientations excitation 
of reactants in bimolecular reactions can give important insights into the microscopic 
dynamics of elementary chemical reactions. The information obtained in such 
experiments can be compared with the results of theoretical calculations of the reaction 
dynamics based on ab initio potential energy surfaces and is also of basic interest to 
improve the kinetic data used in detailed chemical kinetic modelling.


INTRODUCTION
Reactions of free atoms and radicals in the gas phase have attracted numerous investigators during this century mainly for two reasons: Such reactions are among the simplest of chemical processes.They offer favorable conditions for a quantitative theoretical treatment and demonstrate many properties typical of neutral particle reactions.These properties include not only the classical Arrhenius parameters, but also information about the "activated complex" and the "rearrangement" of the reactants during the reactive process, the probabilities of various reaction pathways, the distribution of the reaction energy over the products, and the influence of specific excita- tion of the reacting particles.
On the other hand, the practical use of the fast reactions of atoms and free radicals in combustion processes is one of the oldest of our chemical techniques.Since the classical investigations of Bodenstein and Nernst 2 it has been well known that even the simple combustion of hydrogen does not actually take place in the form given by the stoichiometric relation.Instead, the reaction proceeds through a set of atom and radical reactions.In recent years it has become increasingly clear that the rapid expansion of the use of combustion processes has reached the point that the natural atmosphere is being considerably affected.A thorough knowledge of the elementary steps taking place in explosion and combustion processes is therefore clearly needed.Furthermore, the reactions of free atoms and radicals also play an important part in many photochemically and radiation-initiated processes such as occur inthe lower and the upper atmosphere, in chemical lasers, and in many industrial applications.
The experimental possibilities for studying the dynamics of elemen- tary gas phase reactions in microscopic detail have expanded dramati- cally in recent years as a result of the development of various laser sources.The tunability, polarization, monochromaticity, collimation, coherence and short pulse duration of laser light in a wide spectral range can be used to obtain microscopic information on the molecular course of transferring energy or breaking and reforming chemical bonds.Careful comparison of these experimental results with theo- retical predictions can test potential energy surfaces obtained by ab initio methods and dynamical simulations using quantum and quasiclassical methods.
ROTATIONAL AND VIBRATIONAL ENERGY TRANSFER FROM H2 (v" 1, j" 1) MOLECULES Energy transfer in collisions of hydrogen molecules with atoms and other molecules has been the subject of intensive experimental and theoretical investigations using crossed molecular beam experiments. 3e to the lack of a dipole moment and an electronic absorption spectrum in the vacuum ultraviolet state-selective studies using spectroscopic methods were difficult to perform for a long time, before laser methods became available.Figure 1 shows the excitation and detection scheme of a laser experiment for energy transfer studies of hydrogen molecules.Stimulated Raman pumping is employed to" populate H2 (v" 1, J" 1) selectively in the electronic ground state of hydrogen within a 10 ns laser pulse.The time-dependent popula- tions in rotational and vibrational levels in hydrogen and isotopic modifications can be probed by coherent anti-Stokes Raman spectro- scopy (CARS).Figure 2 shows schematically the experimental arrangement.4 Fifty per cent of the energy output of a linearly pola- rized frequency-doubled Nd:YAG laser (Quanta Ray DCR1A, at 532 nm) is focussed into a Raman cell containing a hydrogen-helium mixture with partial pressures of 20 bar and 10 bar respectively.The helium is used to reduce the pressure-dependent line shift of the Stokes line.Stimulated Stokes Raman radiation is generated in forward and backward directions.Due to the phase conjugation effect in stimulated Raman-scattering, the backward beam displayed a more homo- geneous intensity distribution over the beam cross section and a smaller divergence than the forward scattered beam.Both beams are focussed collinearly into the centre of the reaction cell with a beam waist of about 200/xm diameter for fundamental and Stokes beams.
With this arrangement, the rotational relaxation in pure H2 and He mixtures can be studied.The time dependent CARS signals of H2 (v 1,J 1)andHe(v 1,J 3) are seen in Figure 3a,b.Thesolid line is a simulation based on a "Multi Step Kinetics with Multi Species Transport Model Program.''5 The rate constants obtained from this modelling for the relaxation processes H2 (v 1 J 1) HE(V= 1, J=3) o k13 2.2 x 10 -12 cm3/s and k31 1.4 x 10 -11 cma/s in good Figure 3 Temporal variation of the CARS signals from H2 (v" 1, J" (a); and H2 (v" 1, J" 3) (b); due to rotational energy transfer and diffusion processes.Solid lines: simulations.agreement with measurements using LIF-Spectroscopy in the VUV spectral region for H2 detection. 6he rotational energy transfer rates of H2 (v 1) molecules are about an order magnitude higher than similar rates in the vibrational ground state measured by ultrasonic sound velocity dispersion 7 and in crossed molecular beam experiments. 8Theoretical calculations using ab initio potential energy surfaces show, however, a significant increase of the rotational energy transfer rates in H2-He collisions if one compares H2 (v 0) and H2 (v 1) molecules.In a similar way the vibrational energy transfer from H2 (v 1) molecules can be studied.Figure 4 shows the time evolution of the HD P(H2) 26,7 tabor p(HD) 41,3 mbar Experiment Calculated Figure 4 CARS signal of HD (v 1) formed in the vibrational energy transfer process from H2 (v 1).
(V 1) concentrations induced by the energy transfer to HD (v 0) from H2 (v 1) molecules.The diffusion of excited H2 and HD out of the CARS beam strongly influences the time evolution of the CARS signal.To estimate the influence of diffusion, an analytical expression for the solution of the kinetic equations coupled with transport processes is required.From such modelling calculations the rate constants for the vibrational energy exchange processes H2 (v 1) + HD (v 0) H2 (v 0) + HD (v 1) / AE 469.4 cm -1 of 1.9 X 10 -13 cm3/s in the exothermal and 1.4 x 10 -4 cm3/s in the endothermal direction are obtained.These rates are somewhat lower than previous theoretical estimations. 1

REACTIVE AND INELASTIC CHANNELS IN THE REACTIONS OF ATOMS WITH VIBRATIONALLY EXCITED MOLECULES
The simplest systems in which the specific effect of a selective vib- rational excitation can be studied are those of reactions of free atoms  ,,,,,   ",',(:,:,i---'  with vibrationally excited diatomic molecules.The various channels for removal of the vibrationally excited molecules BC (v) may be written as As model systems for the competition between energy transfer processes and chemical reactions under non-equilibrium conditions one can use simple thermoneutral halogen atom exchange reactions.
Vibrationally excited HC1 (v) molecules can be consumed by H or D atoms in electronically adiabatic processes either by thermoneutral hydrogen atom exchange, the slightly exothermic hydrogen atom abstraction reaction, or in non-reactive collisions.Figure 5 shows an experimental arrangement for the observation of these elementary processes.HC1 molecules in the vibrational ground state are mixed with atoms in a discharge flow reactor.The decay of laser excited HCI (v) is followed by infrared fluorescence.To distinguish between reactive and inelastic pathways, it is necessary to measure the absolute consumption of reactants and formation of products.This is achieved here by measuring the absolute concentration of the vibrational excited molecules using the rapid equilibration between the HC1 (v) vibrational levels and a measurement of the relative population in the levels v I and v 2 as a function of time.The concentrations of the reacting atoms are followed by time-resolved atomic resonance absorption.Figure 6 summarizes the results for the D / HC1 (v 1) system.The non-reactive relaxation and not the hydrogen atom exchange or abstraction reaction is mainly responsible for the high HC1 (v 1) deactivation rate in contrast to predictions from theoretical calculations using semiempirica112 as well as ab initio potential energy surfaces.13 Further informations on this system can be obtained from experiments using translationally hot H and D atoms to study the individual roles of inelastic excitations and reactive atom exchange processes.It is found that the reactive exchange process has a generally lower efficiency than the T-V process.In the case of HC1, however, the degree of vibrational excitation in the reactive channel is higher.It appears that once the system is following the potential energy surface for reactive exchange and has entered the transition state for reaction, the deposition of energy into higher vibrational states is much more facile.However, the system does not enter the reactive surface as readily perhaps because of some geometrical constraints to overcome the reactive barrier.-T=298K ,H,D(v,:O]/Cl (2P3/2) Figure 6 Experimental results for different channels in the D + HC1 (v 0, 1) reaction.
T (K)  Figure 7 Experimental data for the temperature dependence of the rates for vib- rational relaxation and reaction of HCI (v 1, 0) with O (3p) atoms.
mental and theoretical results in the H(D) / H(D) C1 system is given in. 15round state oxygen atoms react relatively slowly with thermal HCI at room temperature.The rate and Arrhenius activation energy of the reaction has been measured directly by several methods.These measurements show that a single vibrational quantum excitation can deliver enough energy for overcoming the potential energy barrier of the reaction.When HC1 (v 1) molecules are generated in the flow system by absorption of the laser pulse, the decay of HCI (v 1) is significantly accelerated in the presence of oxygen atoms.However, the data given in Figure 7 indicates that the reactive channel to form OH / C1 gives only a small contribution to the rapid removal of HC1 (v 1) by O (3p).The rate enhancement is much less than the factor exp (Ev (v 1)/RT).Since the Arrhenius pre-exponential factor is not changed significantly by vibrational excitation, the contribution of HC1 (v 1) molecules to the thermal reaction is small for most tempera- tures of interest.At 200 K thermal excited HCI (v 1) molecules contribute less than 10-3% and at 2000 K about 10% to the total consumption of HC1 by O (3p) atoms.As shown by quasiclassical trajectory calculations, the remaining thermal activation energies for HCI (v 1, 2) are very similar. 16A theoretical model to explain the effective energy transfer in collisions involving P-state atoms as a result of electronically nonadiabatic curve crossing was given by Nikitin and Umanski.17 As shown in Figure 8, several potential energy surfaces exist for the interaction of O (3p) atoms with HC1 (v).At certain distances a nonadiabatic coupling between the different vibronic states is possible.The approach of the reactants O (3p) and HC1 on a triplet surface followed by a nonadiabatic transition to the singlet HOCI surface as an intermediate complex has been discussed as the possible origin of the potential energy barrier in this reaction.However, the fact that this crossing point appears to be necessarily lower than the saddle point of the lowest triplet surface is of course an artifact of the single coordinate correlation diagram.The experimental results on the reverse C1 + OH (v -< 9) reaction 18 and the observed formation of OH (v 1) from O (3p) + HC1 (v 2) 19 indicate that the chemical reaction occurs predominantly vibrbnic adiabatically on a triplet sur- face and does not proceed through a long-living HOC1 complex.
However, such interpretation of the experimental results on the competition of reactive and inelastic channels in the reactions of vibrationally excited HC1 molecules are still qualitative.More quanti- tative ab initio calculations including electronic nonadiabatic processes should be carried out for these systems.
On the other hand the reaction between a hydrogen atom and a hydrogen molecule provides the simplest system which has been studied now theoretically for more than half a century. 2 As shown in Figure 9 single quantum vibrational excitation of the H2 molecule exceeds the Arrhenius activation energy (Ea) the threshold energy (Eo) as well as the classical barrier height (Ec) of the reaction D + H2.A CARS detection system provides an ideal method for monitoring directly reactants and products in the D + H2 (v 1) reaction.The reaction is followed in a discharge flow system, where the atoms and H2 (v 1) molecules were generated by microwave discharges. 21As shown in Figure 10, HD (v 1) and HD (v 0) molecules are formed in adiabatic and non-adiabatic reaction pathways.Information on the competition of reactive and inelastic channels can be obtained by monitoring the decrease of H2 (v 1) in the presence of D atoms (see Figure 11) corrected for the energy transfer process HD (v 1) + H2 (v 0)--, HD (v 0) + H2 (1/= 1) described above.tal results obtained so far indicate about equal importance of inelastic and reactive channels as well as a predominance of adiabatic over non-adiabatic reactive channels.As shown in Figure 12, these experi- mental results are in good agreement with the predictions of quasi- classical trajectory, 22 semiclassical variational transition state 23 and approximate quantum calculations with the Fixed Angle Reactor Model (FARM) 24 using the ab initio LSTH surface 25 or a new surface based on double-many-body-expansions (DMBE). 23The new DMBE surface has a lower classical barrier of 9.65 kcal/mol compared to 9.80 kcal/mol of the LSTH surface.However, the calculated rate constants for reactions of vibrationally excited hydrogen molecules are somewhat lower than previous results, in agreement with the experi- mental data.In some respects, this agreement with quasiclassical trajectories is surprising since the reaction of H2 (v 1) involves only a small number of state-to-state processes.Similarly good agreement has been obtained between predictions of quasi-classical trajectory calculations and experimental results using translationally hot H and D Hz(v=l) [mTorr] 10-T=1,Tms 1) 0 0 /0 .5'0 60p(D) [mTorr]----- D.Hz(v--1)---HD(v--0,1),H Figure 11 Relative decrease of the HE (v 1) concentration in the D + HE (v 1) reaction as function of the D-atom partial pressure.
atoms with CARS and REMPI detection of the reaction products 26 or molecular beam scattering studies 27 sampling also regimes of higher energy at the potential energy surface.As shown in Figure 12 pro- nounced quantum effects are expected at lower temperatures. 28ACTIONS OF TRANSLATIONALLY HOT HYDROGEN ATOMS WITH OXYGEN MOLECULES Chemical reactions of fast ("hot") atoms for nuclear recoil and photo- lysis processes have long been investigated by analysis of their stable end products.On bombarding Li with neutrons, for example, tritium (3H) atoms with a recoil energy of 2.7 MeV are formed.Collisions then retard these fast 3H atoms to the "chemical" energy range around 20 eV to give a broad, continuous distribution of velocities.Thus the reaction energy cannot be controlled directly.Narrow-bandwidth laser light of high intensity and short pulse length (10-8s), on the other hand, allows high concentrations of atoms with defined velocities to be produced by photodissociation on a short time scale.As an example, the reaction of translationally excited hydrogen atoms H T with oxygen molecules is examined.Despite the large number of elementary reactions taking place in the oxidation of hydrocarbons, the important parameters of the combus- tion process are controlled by relatively few elementary reactions.
Sensitivity analysis, shows that the important parameters such as flame velocity are controlled to large extent by the reaction of hydrogen atoms with oxygen molecules. 29This endothermic reaction leads to the formation of the two reactive radicals and is therefore the most important chain-branching step.As shown in Figure 13 the dynamics of such an elementary reaction with a high energy barrier can be studied in microscopic detail  formation by laser photolysis with time-, state-and orientation- resolved product detection with laser induced fluorescence spectro- scopy.The apparatus is shown in Figure 14.Two antiparallel laser beams are directed coaxially through a flow reactor equipped with a baffle system to reduce the scattered light from the laser photolysis pulse and from the dye laser analysis pulse.The dye laser operates with Rodamine 640 and a frequency doubling KDP crystal to generate a pulse in the 306-311 nm region to probe OH radicals by laser induced fluorescence.Fluorescence light is then detected as a function of the dye laser wavelength through emission optics and a filter transmitting between 240 and 390 nm and by a photomultiplier.
Figure 15 gives few examples of the OH nascent rotational state distributions at different collision energies. 3The major part of the relative translational energy of the reactants is converted into rotational energy of the product OH in agreement with the results of quasi-classical trajectory calculations. 31he observed rotational energy distributions give interesting micro- scopic details on the molecular dynamics of these elementary steps.Spin-orbit and orbital-rotation interactions in the OH radical cause fine structure splittings for each rotational level.Each of these fine structure levels can be probed by different rotational subbands.The equally populated.However, as shown in Figure 17a, the A-doublet fine structure state show a clear preference for the lower-energy 1-I + (A') component.The experimental results show that breakup of the reaction complex generates forces in a plane containing the bond to be broken.The OH radical rotates in that plane and JOH is perpendicular to it and to the broken bond.This picture is consistent only with a preferential planar exit channel in these reactions.This could also be directly demonstrated by using polarized photolysis and analysis laser beams.
For experiments using polarized photolysis and analysis laser beams both lasers were linearly polarized (ca.95% polarization) by using 10 Brewster quartz plates respectively (rack-polarizer).Both light beams are then directed through 3,/2 plates so that the electric vectors of both lasers can be adjusted independently to any desired angle.The polari- zation experiments are based on measuring the distribution of ori- entations of the OH angular momentum vector J by using the polarized dissociation and analysis laser.OH fluorescence intensity is observed with the electric vectors of both lasers ED and EE parallel and perpen- dicular to each other.Dissociation of HBr at 193 nm to H + Br (2p3/2) is induced by a perpendicular transition, so that the H atom flight direction is ligned with a sin2-distribution along ED, i.e. vn .LED preferentially.Figure 16 shows the variation of the OH-Q116 (v" O) fluorescence intensity with polarization of the dissociation laser ED relative to analysis laser EE.The observed preference JoH ED .L VH can be explained by restriction in the possible reaction geometries at high collision energies.Trajectory calculations show that the H + O2 reaction occurs essentially in a plane at high collision energies. 31From that we expect Jori .L vn for randomly oriented 02 molecules.The transition moment/XE of Q-lines 26 is perpendicular to the OH rotation plane (llJon) for high OH rotational states.Thus we get maximum OH excitation probability lEE X EI 2 for #o]lEEIIJon +/-vr resulting in higher fluorescence intensity for EEIIED than for EE .l. ED .32This is also confirmed by analyzing the A-doublet excitation of the OHradicals.The physical difference between the two A-doublet com- ponents 1-I + (A') and I-I-(A") arises from interaction of the electronic spin-orbit momentum with the rotation of the molecule.For fast rotation of the OH radical, the unpaired electron in the p orbital of the oxygen is no longer able to follow the movement of the atomic nuclei.
If thep orbital lies in the OH rotational plane, the electron distribution Figure 16 Variation of the OH-Qll6 (v" 0) fluorescence intensity from the reaction H + O:--OH + O with polarization of the dissociation laser E relative to the analysis laser E.
on the oxygen atom changes, becoming increasingly spherical.In contrast, for a l-I-(A") configuration, the oxygen atom moves in the nodal plane of the p orbital and thus continuous to "see" a dumbbell- shaped electron environment, even for fast rotation.This leads to a splitting of the energies of the 1-I + (A') and l-I-(A") configurations, which selectively increases with increasing rotational energy. 33As shown in Figure 17a at 1.0 eV collision energy, three OH radicals were found in the rI + (A") state for each OH radical in the l-I-(A") state.
This shows that the unpaired electron formed after bond cleavage of 02 stays in an orbital in the rotational plane of the OH radical.During the reaction, most of the HO2 complexes do not rotate out of the initial plane, because of the short reaction time at high collision energies (see Figure 17b).The reaction H + 02 is known to take place adiabatically on the ground-state potential surface of the HO2 (2A") radical.Experi- mentally a total reaction cross-section of 0.42 + 0. is found. 34The theoretical reactive cross-section obtained under these 31 35   conditions by quasi-classical trajectory calculations on the Meiius- Blint 36 surface is 0.38 2. These numbers cannot be compared directly, because the multiplicity of the 2A" surface of HO2 is not taken into account.The observed discrepancies may be attributed to a reduction of calculated reaction cross-section due to a "rigid" char- acter and a barrier of 8 KJ mo1-1 in the Melius-Blint surface for dissociation of the HO2 in the reaction HO2 + M H + 02 + M. 37 Later calculations 38'39 reduce this barrier.Also for the reaction (-1) O + OH O + H the Melius-Blint surface apparently overestimates the long-range O-OH attraction, while the Quack-Troe interpolation scheme 37 leads to better agreement with the experimental values at low temperature if the two lowest electronic states of the HO2 radical are taken into account.Calculated rate coefficients obtained by using this theoretical cross sections from the surface 36 are in agreement with shock tube measurements for kl by Schott. 4However, as shown in Figure 5, recent shock tube experiments 41'42 using time-resolved atomic resonance line absorption give higher values for kx in agree- ment with the reactive cross sections obtained in state selected experiments.34 This example shows that even for a very simple radical reaction in combustion more work has to be done on the potential energy surface to obtain a satisfactory agreement between the results from quantum chemistry and state selective and thermal experiments.

Figure 1 Figure 2
Figure1Excitation and detection scheme for energy transfer studies of hydrogen molecules.

Figure 5
Figure5 Discharge-flow system for simultaneous time-resolved detection of the con- centration of reacting atoms and vibrationally excited HCI (v) molecules.

8
Chemical reaction and vibrational deactivation ol HCI (v) in collisions with O atoms.

Figure 12
Figure12 Comparison of theoretical predictions and CARS measurements of the rate constants for the D + H2 (v reaction.

Figure 13
Figure 13 Production of fast hydrogen atoms by excimer laser photolysis and product detection by laser induced fluorescence.
14A detailed of experi-