Isotope-selective infrared multiphoton dissociation of CF3Br in a supersonic free jet

The carbon-isotope selectivity in the multiphoton dissociation of CF₃Br is studied in the collisional region of supersonic free jet. The isotopic abundance of¹²C and¹³C in C₂F₆ formed by recombination of the dissociation products is measured with a quadrupole mass spectrometer. An enrichmet factor of 9.4 is obtained for¹²C with the 9R(30)CO₂ laser line while the factor of 6.9 is obtained for¹³C with the 9P(16) line.     

Observations on the spectral dependence andT/D isotope selectivity in the CO2 laser multiple-photon dissociation of trifluoromethane

Pulsed CO₂ laser multiple-photon dissociation of CTF₃ (v ₂ mode) bathed in argon and CDF₃, CHF₃ or CCl₄ is examined as a function of laser wavelength (9.2–9.6 m) and fluence. The dependence of the dissociation profile on wavelength is analyzed and comparisons are made to prior work. The single-stepT/D enrichment factor for infrared photolysis of trifluoromethane is measured; potential practical isotope separation is discussed. Pulsed infrared laser photolysis of CTF₃ (v ₅ mode) using a 12 m NH₃ laser is also attempted.     

Isotopically selective IR multiphoton dissociation of 1,3,5-trioxane

Searching for oxygen-isotope selectivity, we investigated the infrared multiphoton dissociation of the formaldehyde trimer around 10 µm, in a range where the molecule has a degenerate and a non-degenerate CO stretch vibration and a degenerate CH₂ deformation vibration. In the region of the two latter, the wavelength dependence of the dissociation yield exhibits sharp structures. They were assigned ton-photon resonances (n = 2, 3, 5) by the help of the IR spectrum in the fundamental and overtone region. The O and C selectivities were very small (1.05) near the non-degenerate CO stretch band, but surprisingly large (2–4) in the CH₂ deformation, which has no isotopic shift. The selectivity is not controlled by the first (n-photon) excitation step, but only by a later step. Its assignment is attempted. The pressure dependence of the dissociation is peculiar: Only after an initial decrease, the yield exhibits the usual rise with pressure. We attribute the decrease to a relaxation which perturbs the two-photon resonance.     

Deuterium separation by multiphoton dissociation of dichlorofluoromethane

Absorption and dissociation probabilities of CHCl₂F and CDCl₂F were investigated by a pulsed CO₂ laser in the wavenumber region of largest selectivity for deuterium. The absorption of CHCl₂F, which is a difference band, can largely be suppressed by cooling. At 200 K and at 920 cm^–1 absorption selectivities up to 4000 were found by extrapolation. In the presence of buffer gas, CDCl₂F can be multiphoton excited nearly like a linear absorber (harmonic oscillator). This is interpreted by a nearly resonant collisional relaxation v₇ to v₂ and by the smallness of the cross anharmonicity x₂₇. The dissociation selectivityS was 24,000 at natural abundance. Such large values were measured by a chromatographic method.S depends only onp _D, the partial pressure of the deuterated species. This dependence is approximately ln Sp _D ^–1 . It can be rationalized by considering only the average energy transferred to the nonresonant molecules by collisions with CDCl₂F. The above functional shape is related to an Arrhenius type law. Comparison with trifluoromethane for D separation shows that CHCl₂F has primarily two advantages: its rapid H-D exchange with water and the less stringent requirements of laser energy and pulse length.A preliminary account of this work has appeared in Quantum Electr.2, 13 (1985) (in Chinese)     

Infrared multiphoton dissociation of13CF3Cl induced by CO2 laser pulses

Selective infrared multiphoton dissociation of¹³CF₃Cl induced by CO₂ laser pulses adjusted on = 1071.9 cm^–1 has been studied in the energy rangeE between 0.5 and 2 J per laser pulse or fluence range between 5 and 25 J per cm², and in the pressure range between 0.10 and 60 Torr. The effect of these parameters on the isotopic selectivity of the dissociation gives information on the rotational relaxation constants. As for the dissociation probabilities, they vary exponentially withE ^–1. The applicability of such an Arrhenius-type relation is discussed and semi-quantitatively justified.     

Infrared-laser-pulse control of bond- and state-selective excitation, dissociation and space quantization: application to a three-dimensional model of HONO2 in the ground electronic state

Selective control over the vibrational excitation and space quantization of the dissociation fragments by optimally designed linearly polarized and shaped infrared (IR) laser pulses of the picosecond (ps) and subpicosecond duration is demonstrated by means of quantum-dynamical simulations within the Schr?dinger wave-function formalism for a three-dimensional (3-D) model of HONO₂ in the ground electronic state, wherein the OH and the ON single-bond stretches are explicitly treated, together with the bending angle between them, on the basis of the ab initio defined 3-D potential-energy surface and dipole function. The high-lying zeroth-order vibrational states of the OH bond are prepared selectively both below and above the dissociation threshold of the ON single bond, and demonstrate a quasi-periodic oscillatory behaviour, manifesting intramolecular vibrational energy redistribution (IVR) on the picosecond timescale. Selective breakage of the ON single bond in HONO₂ with more than 97% probability is demonstrated, along with control of the space quantization of the dissociation fragments: the OH fragments rotating clockwise, OH(c), and anticlockwise, OH(a), are prepared selectively, with the OH(a)/OH(c) branching ratio being as high as 10.975. The results obtained show that optimally designed strong and short IR-laser pulses can compete against IVR and manipulate vibrational excitation and dissociation of polyatomic molecules. Received: 3 November 1999 / Published online: 13 July 2000     

Laser multiphoton mass spectroscopy of organometallic compounds: State selective and mass resolved detection of neutral fragment tellurium atoms from C2H5TeC2H5

Typical features in laser multiphoton ionization of organometallic compounds are well evident in the case of the diethyltelluride C₂H₅TeC₂H₅ molecule. The use of a tunable dye laser coupled with time-of-flight (TOF) mass spectroscopy has allowed to establish that a large amount of tellurium is eliminated from the parent molecule as a neutral atom either in its ground or low excited states. Sharp two- and three-photon atomic Te resonances, which give origin to extraproduction of Te⁺ ions, have been identified in the optical spectra measured by varying the laser wavelength.     

Effect of gas pressure and pulse length on the isotopically selective multiphoton dissociation of CF3Cl under CO2 laser pulses

The multiphoton dissociation of CF₃Cl induced by TEA-CO₂ laser pulses has been studied in a focused beam geometry. TheR(10) [00°1–02°0] ( ) laser line was used, so as to dissociate preferentially the minor isotopic component¹³CF₃Cl. The isotopic selectivityS and the dissociation probability per pulse ω were measured in the pressure range between 0.25 and 8 Torr. With short laser pulses (90 ns FWHM),S is found to increase slightly with gas pressure up to 2 Torr, and ω, to increase almost linearly over the whole pressure range studied. A schematic model is proposed which satisfactorily explains these results if the transition rates across the energy level spectrum of the CF₃Cl molecules are assumed to increase with gas pressure.     

Effect of rotational relaxation on the yield of the multiphoton dissociation of CF3Cl and CF3Br under TEA-CO2 laser pulses

The yield of the multiphoton dissociation of CF₃Cl and CF₃Br induced by TEA-CO₂ laser pulses has been studied in the pressure range between 0.25 and 8 Torr, the laser wavelength being chosen so as to excite preferentially the minor isotopic components¹³CF₃Cl and¹³CF₃Br. For both compounds the dissociation probability is found either to increase almost linearly or to decrease monotonously with gas pressure, according as the laser beam is focused or unfocused, respectively. This behaviour is explained by rotational relaxation effects, and a value of 22ns·Torr for the rotational self-relaxation time of CF₃Cl is obtained.     

Enhanced cohesion of photo-oxygenated fullerene films: A new opportunity for lithography

The irradiation of sublimed fullerene (C₆₀ and C₇₀) thin films with ultraviolet light in an oxygen-rich ambient has been found to lead to a substantially increased cohesive energy in the fullerene solid. The decreased solubility and lower vapor pressure of the phototransformed material enables wet (organic solvents) or dry (thermal or photon-induced sublimation) development of photo-defined negative images. One micrometer wide lines with good edge definition are demonstrated. X-ray, infrared, optical absorption, and high performance liquid chromatography reveal that photo-oxygenated C₆₀ retains its fcc crystal structure but with a substantial fraction of the C₆₀ molecules modified with carbonyl (C=O) bonds.     

Two-photon predissociative fluorescence of H2O by a KrF laser for concentration and temperature measurement in flames

Two-photon laser-induced predissociative fluorescence (LIPF) of H₂O is examined as a potential measurement technique of H₂O concentration and temperature in flames. Two-photons of 248 nm light from a narrowband KrF laser excite H₂O to the highly predissociative state in a hydrogen-air flame. The subsequent bound-free emission is observed from 400–500 nm in the flame at temperatures of 1000–2000 K and is found to be free of fluorescence interference from other flame species. This LIPF signal is not affected by collisional quenching due to the short lifetime of the predissociative state (2.5 ps). Broadband laser dispersion spectra, narrowband laser dispersion spectra, laser excitation spectra and probability density functions of the H₂O fluorescence are obtained in the hydrogen flame. The H₂O LIPF signal is found to be temperature sensitive and a two-line LIPF technique is needed for concentration and temperature measurement. The accuracy of a two-line LIPF technique for H₂O concentration and temperature measurement is determined.     

Vibrational resonances of non-rotating HCN in the excited electronic state

A multi reference internally contracted configuration interaction (MRCI) method is used to generate the potential energy function (PEF) of the excited electronic state of HCN molecule. The analytic representation of the PEF is employed to calculate complex eigenvalues (resonance positions and widths) by a discrete variable representation (DVR) of the Hamiltonian for the non-rotating (J =0) molecule. The computational method used is a variant of the filter-diagonalization technique based on a recursive polynomial expansion of the absorbing-boundary-conditions (ABC) Green operator. Reasonable agreement with existing experimental data is found. Received 27 July 1999 and Received in final form 18 October 1999     

Gas-phase photolysis of tungsten hexachloride

The laser-induced decomposition of WCl₆ in the gas-phase is investigated by means of absorption, Raman and laser-induced fluorescence spectroscopy. With visible Ar⁺-laser radiation dissociation of WCl₆ into WCl₄ and Cl₂ has been observed. Further decomposition can be achieved in the presence of H₂ employing ultraviolet Ar⁺-laser radiation at 360 nm. A complete reduction to W requires even shorter wavelengths. The experimental results are analyzed on the basis of model calculations. Implications on the Laser-induced Chemical Vapor Deposition (LCVD) of W are discussed.     

Above-Threshold Dissociation of HD+ in Femtosecond Laser Field

The above-threshold dissociation （ATD） of the HD＋ molecular ion in femtosecond laser field is investigated theoretically. The energy-dependent distribution of the dissociated fragments is calculated using an asymptoticflow expression in the momentum space. The calculations show that the ATD of HD＋ is sensitive to the initial vibrational level of ground electronic state. Multiphoton ATDs can be observed in the dissociation processes. The dynamics phenomena are interpreted by using the concept of light-dressed potential.     

A Theoretical Study of Super-Excited States of F2

In the framework of quantum defect theory, we study super-excited states of F2 molecules which can dissociate into F^＋ （^3P2,1,0） and F^-（^1 So） ion-pair. Based on our calculation, we present a vibrational resolved assignment of the high precision photofragment yield spectra for F^- from the F2 ion-pair production.     

Backward enhanced emission from multiphoton processes in aerosols

We have investigated, both theoretically and experimentally, multiphoton-induced processes in aerosol particles using femtosecond laser pulses. More specifically, we have demonstrated that both multiphoton (1, 2 and 3 photon)-induced fluorescence (MPEF) and laser-induced breakdown (LIB) emissions are strongly enhanced in the backward direction. The backward enhancement increases from 1.8 to 35 (emission ratio between the backward direction and 90°) with increasing non-linear process order n. Application to non-linear lidar of biological aerosols is discussed. Received: 24 April 2002 / Revised version: 3 June 2002 / Published online: 2 September 2002 RID="*" ID="*"Corresponding author. Fax: +33-472/431507, E-mail: wolf@lasim.univ-lyon1.fr     

Photofragment space distribution in the photodissociation of NaI in the spectral range 315–370 nm: the role of molecular axis rotation

The first determination of translational anisotropy parameters β in the photodissociation of NaI molecules in the spectral range 315–370 nm is reported. The anisotropy parameters were determined by the analysis of Doppler resolved absorption profiles of Na(²S_1/2) atoms produced in the photodissociation of NaI by linearly polarized light. The profiles were recorded for two orientations of the photolysis light: parallel and perpendicular to the direction of the probe beam. The value of the parameter β was obtained from a simultaneous fit of the profiles. The role of the rotation of the parent molecules on the branching ratio between parallel and perpendicular transitions in NaI during dissociation is discussed.     

Measurements of multiphoton ionization coefficients with ultrashort ultraviolet laser pulses

Multiphoton ionization coefficients of atmospheric gases were measured with ultrashort ultraviolet laser pulses. The values were obtained using two different experimental setups and the pressure range covered four orders of magnitude. The coefficients were pressure-independent and consistent with numbers predicted by the nonresonant Keldysh theory. Received: 13 June 2000 / Revised version: 5 October 2000 / Published online: 7 February 2001     

Nonadiabatic fragmentation of excited molecular ions: An exploratory study for

The photodissociation of was studied in a one-dimensional approximation, with the aim of understanding the nonadiabatic features of the fragmentation dynamics. In the collinear arrangement of the system, the three lowest excited surfaces (states) interact via two avoided crossings outside the Franck-Condon region, and they are strongly coupled radially by nonadiabatic terms. Electronic transition probabilities for the process , with in one of the three lowest electronic states involved in the fragmentation, were calculated using the semiclassical multichannel S-matrix within the half-collision approach to photodissociation. The reliability of the semiclassical theory, for treating multichannel nonadiabatic processes was analyzed, and inelastic cross-sections for the three processes of electronically selected fragmentations were calculated. The structure found in the calculated absorption lineshapes reveals the marked influence of the nonadiabatic couplings between excited states in the fragmentation dynamics of this molecular ion. Received: 6 March 1998 / Revised: 7 March 1998 and 17 June 1998 / Accepted: 23 June 1998     

Infrared multiple-photon dissociation of CDCl3 induced by a TEA CO2 laser

Infrared multiple-photon dissociation (IRMPD) of CDCl₃ was studied using a tunable TEA CO₂ laser. Effects of number of irradiation pulses, wavelength and energy fluence as well as of sample pressure on the reaction yield are reported.