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Photodissociation processes are examined using a two photon technique for hydrogen atom detection. A birds-eye view of a photodissociation that produces hydrogen atoms is produced rapidly. A number of simple one-color experiments are presented which involve the use of a 205.14 nm photon and the absorption of two additional photons to investigate the resulting hydrogen atom. The benefits and limitations of the use of a one-color experiment using identical pump and probe are discussed.

Research was conducted to examine the H, CH2CHO, CO and OH products of the reaction of O(3P) atom with alkenes using laser-induced fluorescence under single-collision conditions. Near room temperature Boltzmann distributions were used to characterize the CO and OH rotational state populations. Results indicate that chemical reactions that do not involve the sie chains such as release of H atoms or breakup of HCO depend sensitively on the length of the chains.

Relative yields of HCO were determined when O reacts with different alkenes.

Cs atoms when excited to their 7 [SUP2]P, 8 [SUP2]P, and 9 [SUP2]P states have sufficient internal energy to be able to react with H[SUB2] molecules forming CsH and H and they do so. Cs9 [SUP2]P has sufficient energy to react with alkanes; in fact, this does not occur. Instead an efficient n changing but l conserving collision occurs producing Cs atoms in the lower [SUP2]P states. This phenomenon is shown to result from the fact that in the Rydberg state the valence electron is remote from the Cs[SUP+] ion core. Cs9 [SUP2]P does react with propene removing an allylic H atom to form CsH. [ABSTRACT FROM AUTHOR]

Focuses on the irradiation of 2,3,4,5,6-pentafluorotoluene (PFT), trimethylamine and methyl bromide by weak 193.3 nm light. Release of hydrogen atoms as the major channel for PFT and as minor channel for the other two; Implications of the low average translational energy release and the isotropic angular distribution on the location and state of the dissociation.

The theoretical work on the interpretation of the parameters that were extracted from radio and microwave frequency spectroscopy performed by Richard Bersohn at Cornell, is presented. His various other experiments and the description are divided into subjects, which would help to follow science.

When a molecule is dissociated through separate excitations by one or two polarized photons the fragment distribution is forward-backward symmetric with respect to the polarization axis. However, if the molecule is simultaneously excited by optical electric fields, E(ω) and E(2ω), the amplitude for excitation is a sum of an amplitude for absorbing one photon of frequency 2ω and an amplitude for absorbing two photons of frequency ω. When the sum is squared to obtain the fragment angular distribution, there is a cross product which depends on the phase difference ΔΦ=Φ2ω-2Φω. With suitable adjustment of the relative intensities at the two frequencies large unidirectionality can be obtained. The left-right anisotropy on the polarization axis depends on the symmetry of the transitions, real and virtual. In the most favorable case 97% of a particular fragment is ejected in one hemisphere and only 3% in the other. In a less favorable case 60% of the fragment is ejected in one hemisphere. [ABSTRACT FROM AUTHOR]

The phenomenon of interference of the amplitude for absorption of one photon of frequency 3ω and the amplitude for absorption of three photons of frequency ω was theoretically predicted by Shapiro, Hepburn, and Brumer. The interference was demonstrated experimentally by varying the relative phase between the tripled frequency photon and three photons with the fundamental frequency by the groups of Elliott and Gordon in atoms and small molecules. In order to see how general this phenomenon is, five compounds were studied, ammonia, trimethylamine, triethylamine, cyclooctatetraene, and 1,1-dimethylhydrazine. CH3I was used as the tripling gas for light in the range 604–600 nm. Interference was observed in all cases. The last four compounds have low ionization potentials and interference was observed between a 3+1 and a 1+1 ionization process with a maximum modulation of 22%. NH3 with a higher ionization potential requires absorption of 3+2 or 1+2 photons and exhibits a maximum modulation of 33%. We conclude that molecular size is no obstacle and that as long as a molecule has sufficiently strong absorption at the tripled frequency, and sufficient vapor pressure, and the laser fundamental beam is very strong, phase control of interference is observable. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A new method is proposed for the measurement of translational energy distributions in which a species with three energy levels is excited by two successive transitions with perpendicular laser beams. Simulations of the method are carried out for the photodissociation of H2 O and a pilot experiment was performed on Li atoms dissociated from LiI vapor. [ABSTRACT FROM AUTHOR]

A homologous series of methylsubstituted benzenes and pyrazines are excited with an energy E by a short laser pulse following which, typically on a time scale of several hundred nanoseconds, hydrogen atoms are released from the methyl groups. The rate constants k(E) for the formation of the H or D atoms were determined from the time dependence of the laser-induced fluorescence (LIF) signals, and their velocity distributions were measured from the shapes of their LIF excitation curves. The velocity distributions were Maxwellian with translational temperatures remarkably close to the vibrational temperatures, typically 2000–3000 K, of the excited molecules assuming internal equilibration of the large energy E among the large number of internal modes. The logarithm of the rate constants obtained with 193 nm excitation plotted against 1/TV yields a straight line with an activation energy of 97 kcal/mol. A Rice–Ramsberger–Kassel–Marcus model invoking a transition state with several soft modes accounts very well for the rates of dissociation and for most but not all of the average kinetic energy of the released atoms. [ABSTRACT FROM AUTHOR]

Three reactions of the type X+YCN→XY+CN (X, Y=H, H; H, Cl; F, H) were studied by probing the internal state distribution of the CN product. The fraction of the available energy released as CN rotation is very small consistent with theoretical predictions that these reactions are collinear abstractions. At higher incident kinetic energies of the X atom a larger fraction of the available energy is released into rotation, suggesting that the reactive collision can take place at larger angles of attack. In the reverse reaction CN+HD→HCN(DCN) +D(H) using rapidly rotating CN as a reactant the [H]/[D] ratio is 1.4±0.1. [ABSTRACT FROM AUTHOR]

This is the second in a series of papers on the reaction of O(3P) with alkynes in which the internal state distribution of some products are studied. The first paper dealt with acetylene whose two product channels are CO+CH2 and H+HCCO. The present paper deals with the reactions of a series of higher alkynes; however, just the CO release and the H atom release channels were studied. The CO product was rotationally and vibrationally cold in every case. We therefore infer that, except possibly for acetylene, the initial ketocarbene undergoes intersystem crossing to a singlet state and isomerizes to a substituted ketene which then dissociates through a linear C–C–O transition state. The absence of CO vibration energy implies that the energy taken from the initially formed C–O bond to facilitate a 1,2 migration is not returned. The large H atom translational energy implies that the H atom is released simultaneously with the formation of a radical of high resonance energy. Finally, the CO and H atom yields decrease in the longer alkynes, presumably because the dominant reaction channel becomes C–C bond breaking leading to radical pair formation. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The reactions of Cs(n2PJ), n=8, 9, J=1/2, 3/2 with H2 were studied by laser induced fluorescence of the nascent product. No difference was found in the reactivity of the J=1/2 and 3/2 states. The energies available to the reaction products were 11.5 and 16.9 kcal/mol for the n=8 and 9 states, respectively. CsH was found in the v=0 and 1 states but could not be detected in any higher vibrational state. The v=0/v=1 population ratios were 1/0.33 (n=8) and 1/0.42 (n=9). The products rotational temperatures were approximately the same as the ambient temperature of the reaction cell. Thus about 90% of the available energy is released as translation. From these data the following picture is derived. The mechanism is a primarily collinear abstraction and not an insertion. The large translational energy release is caused by a sudden switching on of repulsion when the H atoms are still close to each other. The reaction involves electron transfer, but at Cs–H distances not far from the equilibrium bond length and is therefore not a harpoon reaction in the usual sense. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The phase control of molecular absorption, first proposed by Shapiro, Hepburn, and Brumer was accomplished with CH3I using the technique of Chen, Yin, and Elliott. Red light (λ near 603 nm) was focused in a cell containing CH3I gas at a pressure around 1 Torr. The emerging light, a coherent mixture of the fundamental and the third harmonic (λ near 201 nm), was refocused on a molecular beam of CH3I and multiphoton ionization was detected. Ionization was by two simultaneous processes: a uv photon absorption followed by the absorption of two red photons (1+2 process) and an absorption of three red photons followed by the absorption of two more (3+2 process). Because of the sharp resonant Rydberg transition at 201 nm, the one and three photon matrix elements dominate the transition. Interference between the amplitudes of these two paths was demonstrated by varying the pressure and, hence, the index of refraction of the Ar gas in a tuning cell where the two light beams were refocused by two spherical mirrors. The modulation depth varied with wavelength but had a maximum of 75%, the largest modulation so far observed in this kind of experiment. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

In this paper, we examine how vector properties of photodissociation with a coherent laser pulse evolve in time during and after the excitation process. In particular, we define and explore the transient anisotropy parameter β(t). We apply our formulation to the case of the predissociation of the NaI molecule. We predict that under certain conditions, β(t) displays a beating phenomenon, in which the system oscillates periodically between a parallel-type and a perpendicular-type angular distribution. Depending on the excitation pulse, this ‘‘vectorial’’ beating may be accompanied by a faster beating similar to the ‘‘scalar’’ vibrational beating observed in the past in the NaI system. It is shown that the transient anisotropy beating may be used to obtain experimentally the time-dependent average interfragment separation, and that different J states are characterized by different interfragment separation curves. [ABSTRACT FROM AUTHOR]

Mercury atoms were excited to the 6 3P1 state with a pulsed laser and a lamp at 253.7 nm in the presence of H2, D2, HD, and mixtures of these gases. The hydrogen atom reaction products were detected by vacuum ultraviolet (VUV) laser induced fluorescence. The ratio of yields of the two body channel (mercury hydride and a hydrogen atom) and the three body channel [Hg(6 1S0) and two hydrogen atoms] were determined by combining the H/D atom ratios and a literature value of the HgD/HgH ratio from reaction with HD. Assuming that the sum of the two yields is unity, the yield of the two body channel is for H2 0.63±0.15, for D2 0.79±0.11, and for HD 0.10±0.02 (HgH+D) and 0.65±0.03 (HgD+H). The average kinetic energies calculated by combining these yields with the literature data on internal energy release in the mercury hydride molecules agree with the average kinetic energies measured from the Doppler broadened fluorescence excitation curves. When the E vector of the exciting polarized light was rotated there was no effect on the hydrogen atom spectrum proving that the hydrogen atom velocity distribution is isotropic. When the mercury atoms were excited with circularly polarized light to the J=1, MJ=1 state, the hydrogen atoms exhibited no spin polarization. Electron spin resonance (ESR) signals observed when irradiating a mixture of Hg and hydrogen molecules were shown by isotopic effects to be due to hydrogen atoms dissociated from mercury hydride molecules. [ABSTRACT FROM AUTHOR]

The H, D, and Cl atoms from the photodissociation of HCl and DCl at 157 and 193 nm were detected by laser-induced fluorescence (LIF) in the vacuum ultraviolet region. The Doppler profiles of the H and D resonance lines at 121.6 nm in the LIF spectra indicate that the absorption of HCl and DCl at 157 nm is a mixture of perpendicular and parallel transition (A 1Π, a 3Π0+←X 1Σ+), while that at 193 nm results almost solely from the A 1Π←X 1Σ+ transition. The fine-structure branching ratios, are measured for the chlorine atom by LIF. Combined with the previous results, the branching ratios [Cl*(2P1/2)]/[Cl(2P3/2)] are 0.81±0.09 at 157 nm, 0.50±0.05 at 193 nm for HCl, and 0.23±0.05 at 193 nm for DCl. These results suggest that the nonadiabatic couplings during the breakup of HCl (DCl) in the excited states play an important role in determination of the fine-structure branching ratios. [ABSTRACT FROM AUTHOR]

The reaction H+CD4→CHD3+D is shown to take place by an inversion mechanism. The evidence is as follows. When the H atom has an anisotropic (perpendicular) velocity distribution, the D atom velocity distribution is also perpendicular. For a relative energy near 2 eV, the reaction cross section for H+CD4 is 0.084±0.014 A2 and for H+CH3D is 0.040±0.015 A2. At the same H atom energy, when CH3CD3 is substituted for CD4, no D atoms can be detected. Finally, around 80% of the initial H atom kinetic energy is released as kinetic energy of the D atom showing that the reaction is nearly vibrationally adiabatic. [ABSTRACT FROM AUTHOR]

The dissociation of hydrogen atoms from the methylene group of cyclopentadiene (CP) and indene (ID) excited with a 193 nm photon has been studied by hydrogen atom laser induced fluorescence. The rate of dissociation of IND was 7.4×106 s-1 but that of CP was too fast to measure. The ratio of H atoms to D atoms generated from 5-deuteriocyclopentadiene (5-dCP) was 3.91±0.46. Rice–Ramsberger–Kassel–Marcus theory was used to calculate the rates of dissociation of CP and 5-dCP. The quantum yield for dissociating H atoms from CP was 0.85±0.07. The ejected H atoms have a Maxwell velocity distribution with temperatures which are equal to the vibrational temperatures, 3690 and 2479 K for CP and IND, respectively. The most important result of the work is this confirmation of an earlier finding on a different set of molecules that the translational temperature of the fragments after the dissociation is equal to the vibrational temperature before the dissociation. This is explained by the assumption that the motion of the fast, light hydrogen atom is partly decoupled from that of the heavier, slower atoms. [ABSTRACT FROM AUTHOR]

We present a theoretical synthesis of Zewail’s femtosecond transition state (FTS) experiments. The FTS transients are obtained by convoluting the continuous wave, bound–free, and free–free frequency-resolved spectra with the pump and probe laser pulses. We accomplish this by deriving a simple semiclassical expression for the free–free (as well as the bound–free) matrix elements and regularizing the divergent integrals occurring in the free–free integrals, by subtracting a δ-function singularity. This method allows us to write the FTS transient signal as a coherent superposition of a ‘‘molecular’’ term and an ‘‘atomic’’ term, which shows clearly the effects of the interaction between the on-resonance and off-resonance transients observed in Zewail’s experiments. Our method, which achieves essentially the same accuracy as the purely numerical wave-packet propagation techniques, is much more efficient. It enables an easy exploration of all possible pump–probe configurations and clearly confirms the fact that quantum mechanics does not allow a unique definition of a ‘‘bond-breaking’’ time. In fact, we show that the observed transients are a strong function of the laser preparation and probing. In addition, we demonstrate that the assumption that the initial wave packet is localized after absorption of a 50–100 fs pulse is wrong, and that the classical approximation that there exists a one-to-one correspondence between the probe frequency and the internuclear distance of the photofragments is inaccurate. We show that the parameters obtained by the use of these classical approximations for the free–free spectrum lead to the extraction of the wrong potential parameters which, in turn, leads to a discrepancy between the calculated and measured bound–free absorption spectrum. This discrepancy does not exist... [ABSTRACT FROM AUTHOR]

Alkyl chlorides (R=CH3, C2H5, C3H7, and C4H9), chloromethanes (CHnCl4-n), and chlorofluoromethanes (CFnCl4–n) are photodissociated at 157 and 193 nm. The chlorine atom photofragments are detected by a resonance enhanced multiphoton ionization technique. The branching ratios of the Cl photofragments [Cl*(3p2P1/2)]/[Cl(3p2P3/2)] are almost identical (0.23±0.03) for alkyl monochlorides at 157 and 193 nm and for CH2Cl2 and CHCl3 at 157 nm, while the ratios are rather small (0.10±0.02) for CH2Cl2 and CHCl3 at 193 nm. No discernible isotope effects on the branching ratios were observed when D atoms were substituted for H atoms in chloromethanes. For CCl4, CF3Cl, CF2Cl2, CFCl3 at 157 and 193 nm, the ratios are small (<0.05) irrespective of the number of Cl atoms in the parent molecules. The Doppler profiles of the chlorine photofragments have been obtained and it is found that (a) CF3Cl undergoes a perpendicular optical transition at 157 nm and (b) for halomethanes containing more than two Cl atoms, the Cl photofragment has a Boltzmann distribution in translational energy and an isotropic angular distribution. [ABSTRACT FROM AUTHOR]

The photodissociation of ethylene at 193 nm is studied by laser induced fluorescence of the hydrogen atom product. The quantum yield for producing deuterium atoms from C2D4 is 0.19±0.04. Their average translational energy is 7.3±1.3 and 15.1±1.4 kcal/mol at 193 and 157 nm, respectively. Considerable evidence, both experimental and theoretical, shows that dissociation takes place from a hot internally equilibrated ground state molecule. A remarkable isotope effect is found. The ratio of H atoms to D atoms released from CH2CD2 is 2.2±0.2, whereas that from t-CHDCHD is 1.2±0.1. An RRKM model explains these ratios. For CH2CD2 the vibrational frequencies of the transition state for releasing H atoms are quite different from those for the release of D atoms. In t-CHDCHD, the transition state frequencies are nearly the same; there is a cancellation between differing zero point energies and moments of inertia. [ABSTRACT FROM AUTHOR]

The exchange reaction H(1 eV)+RD→RH+D, where RD was deuterated acetylene, methylacetylene, ethylene, and propylene was studied by laser induced fluorescence detection of the hydrogen and deuterium atoms. The reaction cross sections were 1.69±0.22, 0.50±0.15, 1.85±0.20, and 1.10±0.14 Å2, respectively. The fraction of the incident relative kinetic energy released as translation was 0.42, 0.34, 0.40, and 0.34. That the addition of a methyl group only slightly reduces the energy released as translation is regarded as proof that the reaction at this energy does not involve a long lived intermediate complex, but is a direct localized displacement. Further confirmation comes from the observation that the velocity of the D atom product of H+C2D2 is correlated with the velocity of the H atom reactant. [ABSTRACT FROM AUTHOR]

In this work we probe an ensemble of energetic hydrogen atoms dilutely dispersed in a bath gas which is at thermal equilibrium. A recently developed laser-induced fluorescence technique allows a quantitative measurement on a nanosecond time scale of the shape of a Doppler-broadened absorption line of hydrogen atoms. A nonequilibrium distribution of H atoms is generated by photodissociation, and the line shape is then measured at a flxed time delay as a function of buffer gas pressure. The buffer gases used are He, Ar, Kr, Xe, O2, and N2. A technique perfected by Shizgal is used to integrate the linearized Boltzmann equation for a hard-sphere potential. The experimental results for the decay of the kinetic energy are fit with cross sections of 10±2, 9±2, 12±2, and 16(+8-4) Å2 for He, Ar, Kr, and Xe, respectively. [ABSTRACT FROM AUTHOR]

When energized sufficiently either vibrationally or electronically, ROH (where R is methyl or ethyl) can dissociate to form H atoms and RO radicals. We have determined the translational energy release (=0.82Eavl ) and angular distribution (β=-0.60±0.03) from the laser induced fluorescence spectra of H atoms produced in the 193 nm photodissociation of CD3OH. We have also determined that the quantum yield for producing H from CD3OH is 0.86±0.10. In contrast, the reaction of O(1D)+CH4 which produces vibrationally excited CH3OH, has a quantum yield for producing H atoms of roughly 0.25 with only 22% of the available energy released as translation. We conclude that although the total available energy is the same in both cases, the dissociation of photoexcited methanol is prompt whereas the dissociation of chemically activated methanol shows some degree of internal vibrational equilibration. [ABSTRACT FROM AUTHOR]

The absorption and fluorescence excitation spectra of jet cooled Cd(CH3)2 and Cd(CD3)2 were measured in the structured region between 230 and 200 nm. The symmetric Cd–methyl stretch and methyl umbrella modes in the second excited electronic state of the latter molecule were found to the 304 and 790 cm-1, respectively. Widths of the 0–0 lines (FWHM) were 101 and 25 cm-1, respectively from which lifetimes of 53 and 265 fs were deduced. A weak fluorescence due to CdCH3 is excited by the absorption of Cd(CH3)2 and an even weaker fluorescence is excited in Cd(CD3)2. The quantum yield of this fluorescence is below 10-4 at the electronic origin of the structured band and increases to about 10-3 above 2000 cm-1 vibrational energy. [ABSTRACT FROM AUTHOR]