Three markets exist for processes capable ot separating hydrogen isotopes: (1) separation ot D from H, (2) separation of T trom D, and, (3) separation of T from H. The selective Multiphoton Decomposition (MPD) process has been proposed for each of these markets and a search has gone on, in several countries, for the suitable working molecule for each of these separations. In this paper the incentives for the separation of hydrogen isotopes are listed, the MPD process is outlined, and the search for suitable working molecules is reviewed.

The multiple photon excitation of UF, at 16 μm has been studied in a number of laboratories, in view of its potential applications in uranium enrichment, using variousischemes : IR+UV /1/, IR+IR /2/. Because of the small isotope shift : 0.61 cm-1, on the ν3 line at 627 cm-1 , adiabatic cooling of UF6 at temperatures lower than 120 K, lg re9ired to get UF6, with a simplified IR spectrum, at reasonably high molecular densities in the order of 10¹⁵ cm^-3.

Optimum irradiation conditions and maximum process flow rates were investigated experimentally in laser isotope of tritium using trifluoromethane as a working substance. The absorbed energy, ε_abs (kJ/mol), and the fractional conversion of CT₃ per pulse, q_T, were measured as functions of irradiation wavenumber, gas pressure and temperature. The results of optimization were shown in contour maps where qT/εabs was adopted as the objective function.

We observed self-focusing and self-defocusing of a TEA CO₂ laser pulse in CDF₃ vapor under different conditions. The experimental parameters that we varied are: the pressure inside the interaction cell, the frequency of the laser line, the intensity and the temporal length of the pulse. We have shown that it is possible to pass from self-focusing to self-defocusing by only increasing the energy of the laser pulse of a fixed temporal length. We propose a physical model using the selection rule of the molecular transition that can explain these experimental results.

Multiphoton dissociation of CF₃C1 and CF3Br induced by CO₂ laser pulses has been studied in the (0.1 - 60 torr) pressure range and (S - 30 Joule/cm2) average energy density range. The laser was adjusted on 1071.9 cm^-1 and 1052.2 cm^-1, resp., so as to excite preferentially the 13C isotopic molecules. The effects of pressure and energy on the isotopic selectivity were measured and a remarkable difference was found between the two molecules. As for the dissociation probabilities, they vary exponentially with the inverse of the energy. The applicability of such an Arrhenius-type relation is discussed and semi-quantitatively justified.

Recent work from our laboratory on zirconium beams is reviewed. Zirconium metal beams have been produced by laser vaporization of solid zirconium targets coupled with supersonic expansion of helium gas. The resultant supersonic metal beam is shown to present an ideal environment for various spectroscopic techniques. The state distribution of zirconium atoms in the beam is obtained from low resolution laser induced fluorescence (LIF) studies. High resolution LIF studies give information on the hyperfine splitting in the ground state of the zirconium-91 isotope. Information on the hyperfine splitting in the excited state is obtained from quantum heat spectroscopy. Low resolution 2 color multiphoton ionization spectroscopy using a XeC1 laser allows isotope separation of all isotopes of zirconium. This same technique, using a tunable UV laser reveals that the previously accepted values for the ionization potential for zirconium are in chemically significant error (2500 cm-1) and permits precise (±0.3 cm-1) measurement of the ionization potential for this element via the observation of long (n=55 to 80) Rydberg series. These metal beams are highly reactive and can be used to produce novel chemical species. The results of two studies in which a reactant is added to the expansion gas are reported here. Zirconium oxide (ZrO), a molecule observed in the emission spectra of cool stars and in laboratory studies at high temperatures, is produced in a low temperature, collision free environment by adding small quantities of oxygen to the expansion gas. Zirconium fluoride (ZrF), a molecule previously unobserved, is produced by the addition of small quantities of CF₄.

The resonant enhanced multiphoton ionization (REMPI) method has been applied to the determination of the energy disposal in the photofragments of NO2. Rovibrational energy distribution and rotational alignment have been obtained for the NO fragment after analysis of the two-photon excitation spectra of some (A ← X) and (D ← X) NO transitions. The velocity distribution of the fragments and its angular dependence have been obtained by time-of-flight analysis of the REMPI ionized fragments in a massspectrometer allowing selectivity and improved resolution (v = 130 m/s).

Multiphoton, two-laser absorption by methanol (Pressure = .4 kPa) has been studied photoacoustically. The P1(6) (3694 cm -1) and P1(7) (3644 cm-1) lines, of an HF laser were used together with three C02 laser lines, 10P(20) (944 cm-1), lUR(16) (973 cm-1) and 9P(20 (1047 cm-1). The two lasers were overlapped spatially, with counterpropagating beam geometry, and temporally with a relative jitter of approximately ± 70 ns. Preliminary results show that when the HF laser pulse (w1) precedes the CO² laser pulse (ω2), with ωl exciting the molecule into the quasicontinuum (QC), there was a two-colour enhancement (TCE) of the multiphoton absorption signal which was considerably red-shifted from the C-0 stretch fundamental absorption maximum. The maximum enhancement observed was for the lUR(16) line which is approximately 60 cm-1 to the red of the C-0 stretch absorption maximum. There was also a significant two-colour absorption observed for the 10P(20) line at moderate CO2 laser fluences (50 J/cm'), whereas there is no one colour signal observed for this line at fluences as high as 100 J/cm4. No TCE was observed when the CO₂ laser (9P(20) line) preceded the HF laser pulse and pumped the molecule into the QC, for either the P1 (6) or P1 (7) HF laser lines. In addition, no dependence of TCE on CO₂ laser pulse intensity was observed when using 10 ns as compared to 60 ns pulses. There is noc enough data, at this point, to say whether there are several regions of varying absorption intensity in the OC or whether there is only one broad red-shifted peak, hot experiments along these lines are continuing.

The infrared multiphoton dissociation (IRMPD) of "neat" hexafluoroethane has been investigated for the first time. The stable products of photolysis are measured by IR absorption spectroscopy with a tunable lead-salt diode laser (TDL), which offers high sensitivity combined with a non-destructive measuring technique. The high sensitivity and resolution of the TDL allow measurement of the major stable products CF4 and C2F4 after irradiation by a single infrared laser pulse. A mechanism of IRMPD of C2F6 is proposed.

One-photon laser excitation and subsequent bulk collisional relaxation of target molecu-les highly dispersed in a heat bath yields information about energy transfer in the bulk system Ca macroscopic time-dependent property], provided the internal energy of the target molecules is monitored as a function of time. If the relaxation of internal energy is ap-proximately exponential, it is possible to obtain the average energy transferred per collision Ca microscopic time-independent property] without knowledge of the collisional transition probability. The implication of exponential relaxation is that the first moment of the collisional transition probability is approximately a linear function of internal energy. These conclusions are illustrated using available laser relaxation data on azulene, benzene and hexafluorobenzene, and are compared with similar data on toluene and two cycloheptatrienes. Some inconsistencies are noted, the probable origin of which is discussed.

Two-color, two-photon resonance enhanced ionization (R2PI-2C) is shown to be a powerful tool for the determination of adiabatic ionization potentials and vibrational frequencies of ground ionic states of polyatomic cations. Results on the laser induced dissociation of phenetole cations prepared by R2PI-2C will be presented.

The decomposition of ethylene induced by a pulsed infrared CO2 TEA laser has been explored at pressures from 500 to 3000 Torr, using the strongly absorbed P(14) line at 949.5 cm^-1. Under these conditions the reaction zone is a thin disc at the front window of the reaction vessel, and the characteristics and behavior of this "thin-disc reactor" are explored.

Using both thermal initiation and laser photolytic techniques, we have been studying important kinetic processes in the partial oxidation of methane. Prompt production of a relatively high concentration of free radicals via laser photolysis makes it possible to separate the thermal initiation step from the subsequent chain propagation steps. If a particular product such as methanol is desired, the conditions (temperature, pressure, and mixture composition) for rapid thermal initiation and optimum yield may differ and laser initiation of the chain provides an exciting potential application for laser-induced chemistry. We report our results on the partial oxidation of methane by oxygen at moderate temperatures and pressures. Using 193 nm photolysis of trace amounts of acetone as a source of methyl radicals, dramatic evidence of chain reaction processes is observed for laser initiation under experimental conditions where thermal initiation is not. significant. Details of the proposed mechanism are discussed as well as application of the technique to technologies for methane conversion to transportable fuels such as methanol.

The production of gas-phase gallium atoms in the photolysis of trimethylgallium has been investigated at 193 nm and at other laser wavelengths. Ground state (42P03/2) and metastable (4 2P01/2) gallium atoms are detected using laser-induced fluorescence techniques. Our results indicate that gallium atoms continue to be produced at long times after the laser pulse. The observed dependences on photolysis laser fluence, trimethylgallium pressure, and buffer gas pressure are consistent with a mechanism in which highly excited gallium methyl radicals undergo unimolecular decomposition to produce gallium atoms. Since this process is observed to happen on the time scale of hundreds of microseconds, these results have important implications for studies of metal deposition and direct laser writing by laser photolysis of organometallic compounds.

The reaction of phosphine and nitrogen trifluoride initiated by a CW CO2 laser has produced several interesting results. Mixtures that are rich in phosphine produced a brownish polymeric film, along with phosphorus trifluoride, hydrogen fluoride, and nitrogen. The infrared spectra of the polymer indicated that N-H, P=N, and P-F vibrational bands were present. It was observed that when hydrogen bonded NH vibrational bands were present in the IR, the polymeric film was slightly conductive. The film could also be made to be conductive by doping with iodine vapors. The polymer film was observed to decompose slowly when exposed to air due to hydrolysis. A visible emission is observed when the phosphine and nitrogen trifluoride mixtures are irradiated. Spectral analysis of this fluorescence showed that the discrete spectra was due to PF, CN, atomic Na, and CaF. Most of the fluorescence was the result of a continuum that extended across the visible into the near infrared. The emission was observed to give simple exponential decay to oscillations with spikes as the nitrogen trifluoride to phosphine ratio was increased. The oscillations have been shown to be due to the shock waves generated by the reaction traversing the cell. The reaction cell was fitted with Brewster angle windows and placed in a hole-coupled cavity to determine if lasing could be obtained. Some gain has been observed from 400 to 800 nm. A comparison of the laser-induced emission to the emission from the phosphine/nitrogen trifluoride flame has also been made.

This is a quantitative experimental study of the geometrical redistribution of resonant molecules in the beam of coherent radiation. The system considered is ethylene and CO₂ laser light.

The production of vinyl chloride from 1,2-dichloroethane by direct XeCl laser photolysis has been studied with respect to by-product formation in comparison with the thermal reaction. Acetylene and Chloroprene, the main products, could be reduced substantially because it was possible to work at low temperatures. Ethylene, a minor compound, increased due to shorter radical chain lengths. The computer model of the reaction system was extended to include the side reaction paths. Control experiments with added products improved the kinetic data of elementary steps.

Using a molecular beam device, experimental multiphoton ionization of the CH₃I molecule is reported. At low pressure (10^-4 torr - 10^-6 torr), we have obtained the current dependence on pressure and laser intensity.

Clusters containing two to several hundred atoms are generated by pulsed laser vaporization of a metal target located in a flow tube reactor. A continuous flow of inert carrier gas entrains and cools the vaporized plume of metal, resulting in rapid cluster growth. Reactant gases are injected into the flow downstream of the target, at a point where cluster growth has finished. After reagent mixing and chemical reactions occur, the products exit the tube into a vacuum, and are formed into a molecular beam by collimators located in successive stages of differential pumping. In the highest vacuum stage the reaction products are identified by pulsed laser ionization and time-of-flight mass spectrometry. Studies to be discussed include measurements of absolute reaction rate constants, hydrogen uptake experiments, laser-induced desorption of adsorbates from clusters, and cluster-catalyzed chemical reactions. Such work provides us with information about the dependence of reactivity on cluster size, the geometrical configuration of cluster binding sites, the thermodynamics of adsorbate bonding, and the mechanisms of chemical reactions on cluster surfaces. These studies are bringing us closer to a detailed understanding of the interactions of metal surfaces with atoms and simple molecules.

The use of tunable infrared lead salt diode lasers as spectrally bright light sources for infrared reflection absorption spectroscopy (IRRAS) at interfaces will be presented. This new spectroscopy permits in-situ, non-intrusive optical probing of the vibrational fingerprint spectrum (500-2000 cm-1) with sub-monolayer sensitivity at metal interfaces and can work through any moderately transmissive ambient. Recent results and instrumental developments will he presented.

Two level systems can interact coherently with infrared laser radiation giving rise to Rabi oscillations. The description of the interaction can in most cases, be cast in terms of the optical Bloch equations, as was shown by Feynmann et al.' Experimentally the interaction may be displayed using apparatus such as that described in reference 2. A molecular beam is generated from a supersonic expansion, crosses the output of a frequency stabilised carbon dioxide laser and impinges on a 2K bolometer. A Stark field is used to tune vibrational transitions of the beam molecules into resonance with the laser: the degree of vibrational excitation can be measured by monitoring the change in resistance of the bolometer as the molecules are tuned through resonance.

In this paper we present the results of a study of medium sized noble gas clusters containing at least one infrared active molecule carried out by means of laser photo-evaporation spectroscopy techniques which make use of sensitive bolometric photofragment detection.1

An instrument for the study of the chemistry and photodissociation spectroscopy of metal cluster ions has been constructed. The instrument incorporates a source using either laser, ion, or fast atom sputtering to generate cluster ions of nearly any material. Radio-frequency ion guides are used to collect, transport, and store the clusters; storage in a buffer gas is used to thermalize hot clusters produced in the source. Studies of the chemistry of aluminum and cobalt clusters are used to illustrate the operation of the instrument.

An apparatus for studying high resolution overtone spectroscopy on a molecular beam has been constructed. With it molecular overtone lineshapes with dramatically reduced spectral congestion and Doppler broadening can be interrogated. A high sensitivity, low temperature bolometer is used to monitor the energy content of the molecular beam. A continuously tunable narrow band laser source, efficiently amplified in a resonant Fabry Perot cavity through which the molecular beam travels, imparts vibrational energy to the molecules. This is then transferred to the bolometer and therefore gives rise to the spectrum when the frequency is scanned. ▵v = 4 overtones of H₂O have been studied. Other molecules have also been considered and will be discussed in terms of the nearby density of dark states.

Raman spectroscopy is a mature field of science that does not need introduction nor rationalization for its use. The majority of the Raman spectroscopy literature is concerned with C.W. high resolution spectroscopy. The Raman Scattering instruments utilize almost exclusively laser light sources, and lately nanosecond and picosecond data on time resolved spectra have made their entrance into the scientific journals. The difficulty in most of these resonance Raman experiments is that they do not provide any more time dependent information than fluorescence. This is due to the limitation that a single laser, quite often, a dye laser and its second harmonic are the only two frequencies available for excitation and resonance probe of the excited state. In this scenario, a Raman Scattering signal is emitted and detected only during the lifetime of the excited state. As the excited state decays to either the ground state or other transient species which absorb at a different wavelength, the resonance with the probe wavelength disappears at the same rate as the population of the excited state decays. This rate of depopulation is also portrayed in an identical fashion. These systems are therefore drastically limited in their use as means for the measurement of the evolution of a chemical intermediate. An additional interesting aspect is that of the understanding of the process itself which is being studied, namely the majority of the research papers presented do not address the possibility of the data depicting stimulated emission gain rather than Raman Scattering.

The synthesis of silicon nitride surface layers on metal substrates by laser-activated deposition of organometallic precursors has been demonstrated. Advantages of this technique include high deposition rates, constancy of composition, and avoidance of toxic or pyrophor ic reactants. The uniform morphology surface layers of up to 20μm thickness contained 2000 Å diameter particles of amorphous silicon nitride. Auger electron spectroscopy and Fourier transform infrared reflection spectroscopy revealed there to be oxygen contamination at the surface. Attainment of a uniform morphology required a specific set of processing conditions. The presence of residual Si-CH₃ bonds within the layers was shown to be reduced by subsequent laser heating. Suggestions are made for obtaining Si3N4 layers of improved purity, also for extending this approach by using a variety of organometallic precursors to obtain SiC and Si0₂ layers.

Molecules in intense fields (I ≥ 10⁹ W/cm2) cannot be treated by perturbative methods. We have developed nonperturbative methods to calculate multiphoton cross sections in molecules based on the coupled equations of quantum collision theory. This enables us to include bound and continuum states, and to bridge the weak field (classical spectroscopy) and strong field (laser induced phenomena) cases. In particular, in the case of diatomic molecules, one can show that strong fields will induce new bound states which can drastically influence photodissociation cross sections. Examples will be presented of calculayions of two photon and three photon effects in intense fields where perturbative approaches no longer apply.

We report the implementation of a technique for the observation of laser-initiated chemical processes using time-resolved Fourier transform spectroscopy. This technique combines state-specific creation of reagents and the high resolution and broad spectral coverage of conventional Fourier transform spectroscopy with 10 microsecond time resolution of the spectral measurement. It has been applied to the study of energy partitioning in the reactions of 0(1D2) atoms with several small molecules in the gas phase at pressures of a few millitorr. Using this technique, collisional deactivation of the reaction products can be eliminated and the initial energy distribution created by the reaction is obtained. In addition, energy transfer rate constants can be measured and secondary reactions can be identified.

Time-resolved excited state triplet-triplet absorption spectra were measured for solutions of 2,5 diphenyloxazole (PPO) and 2,1 napthyl, 5 phenyloxazole (aNPO) in several solvents. Concentration quenching effects due to excimer formation in nonaromatic solvents were observed. A numerical analysis of the experimental results yielded the rate constants for intersystem crossing, triplet quenching by 02, triplet self quenching and the formation of excimers.