Infrared multiphoton excitation of SO2 to fluorescent states

SO₂ molecules excited under essentially collision-free conditions by intense (>20 G/Wcm²) CO₂ laser pulses give off a broad UV—visible luminescence through inverse electronic relaxation. Unlike in larger polyatomics, the infrared multiphoton excitation process is controlled by the laser intensity and not the laser fluence.     

Study of nitrogen isotope-selective two-stage IR + UV dissociation of ammonia molecules

The one-photon IR excitation and subsequent UV dissociation of ammonia molecules selective with respect to nitrogen isotopes were studied. The selectivity of vibrational excitation is achieved by tuning CO₂ laser radiation to resonance with ¹⁴NH₃ or ¹⁵NH₃ molecules. The dependences of the yield of dissociation for each isotopic component and the selectivity on the buffer gas (N₂, O₂, Ar) pressure, the partial pressure of ammonia, and the time of delay between IR and UV laser pulses were established. At low pressures (67–270 Pa) of the isotopic mixture with a ¹⁵N concentration of 4.8%, the dissociation selectivity for ¹⁵N was 17. The mechanisms responsible for the selectivity of IR + UV-initiated dissociation are discussed. The phenomenological model has been developed that takes into consideration the effect of the interisotopic V-V exchange and V-T relaxation on the formation of the yield and selectivity of the two-stage IR+UV dissociation of ammonia.     

Vibrational-translational/rotational and vibrational-vibrational processes in methane/inert-gas mixtures: Optoacoustic phase measurements

We consider the model with kinetic excitation into the quasicontinuum (KEQ) for resonant polyatomic molecules which absorb laser radiation and are surrounded by buffer molecules. KEQ takes place when the resonant molecules in the lower part of the energy spectrum interact weakly with the laser radiation, but the molecules in the quasicontinuum are rapidly excited to still higher energy and dissociate. Under these conditions the collisions of the resonant and buffer molecules lead to excitation of resonant molecules into the quasicontinuum because the population of the quasicontinuum is much less than its thermodynamical equilibrium value. It is found, that the smaller the V-T relaxation time τ_VT, the larger the rate of KEQ and the dissociation rate (if only τ_VT is not too small). Thus, if we change the experimental conditions and decrease τ_VT (for instance, by passing from the heavy buffer gas Xe to the light buffer gas He), for some resonant molecules we may observe that the probability of dissociation increases.     

Electron transfer from laser excited rydberg atoms to molecules. Absolute rate constants at low and intermediate principal quantum numbers

Using mass spectrometric detection of positive and negative ions, we have investigated ionizing reactions of Ne(ns,nd) Rydberg atoms, efficiently excited by resonant two-photon excitation of metastable Ne(3s ³ P ₂) atoms, with electron attaching moleculesBC (BC=SF₆, CCl₄, CS₂, O₂) at thermal collision energies. Absolute rate constants have been determined in the range of low and intermediate principal quantum numbersn(5≦n?30) by utilizing the photoionization signal caused by room temperature black-body radiation and the loss of Ne(3s ³ P ₂) atoms, associated with the laser excitation. Substantially differentn-dependences of the electron transfer cross section have been found for the larger molecules (BC = SF₆, CCl₄) and the smaller molecules (BC = CS₂, O₂). Simple model calculations have been performed to gain new insight into the dynamics of the electron transfer process; forBC = SF₆, our results at lown(5 ≦n ≦ 10) suggest that internal energy conversion in the Coulombic complex Ne⁺ — SF ₆ ^? is important for the formation of the detected ions.     

Photolysis and desorption of adsorbed halomethane molecules under long-wavelength radiation of excimer lasers

The photolysis and desorption of CH₂I₂, CH₃I, CCl₄, CHCl₃, CH₂Cl₂, CF₂Cl₂, and CHF₂Cl molecules adsorbed on fused silica under the action of XeF and XeCl laser radiation absorbed by these molecules have been studied. The desorption of molecules that occurs due to expansion to the long-wavelength region of the absorption spectrum of molecules in the adsorbed state, compared to the gas phase, predominates. The laser desorption is characterized by a strong nonlinear dependence on the density of radiation energy. Depending on the relationship between the laser radiation wavelength and the spectrum of electronic states of molecules, photolysis is observed upon absorption of either one or two photons. At an increased fluence of laser radiation energy, the one-photon detachment of the primary CH₂I fragment from the CH₂I₂ molecule changes into the three-photon process. A similar behavior is revealed for the desorption of CH₃I molecules from clusters formed on the surface in multilayer adsorption coverages.     

Two-laser studies of E → V energy transfer reactions involving CO and electronically excited I2, ICl and NO2

The relaxation of electronically excited I₂, ICl and NO₂ by CO has been investigated using a dye laser for electronic excitation (in the 590 nm region) and a cw CO laser for measuring the extent of CO product vibrational excitation. The CO molecules formed in these quenching reactions were found to carry very small fractions of electronic energies, in sharp contrast to the results observed in other E → V transfer reactions involving atomic species which carry comparable amounts of electronic energies.     

Quantum State-to-State Vacuum Ultraviolet Photodissociation Dynamics of Small Molecules

The present review focused on selected, recent experimental progress of photodissociation dynamics of small molecules covering the vacuum ultraviolet (VUV) range from 6 eV to20 eV. These advancements come about due to the available laser based VUV light sources along with the developments of advanced experimental techniques, including the velocitymap imaging (VMI), H-atom Rydberg tagging time-of-flight (HRTOF) techniques, as well as the two-color tunable VUV-VUV laser pump-probe detection method. The applications of these experimental techniques have allowed VUV photodissociation studies of many diatomic and triatomic molecules to quantum state-to-state in detail. To highlight the recent accomplishments, we have summarized the results on several important molecular species, including H₂ (D₂, HD), CO, N₂, NO, O₂, H₂O (D₂O, HOD), CO₂, and N₂O. The detailed VUV photodissociation studies of these molecules are of astrochemical and atmospheric relevance. Since molecular photodissociation initiated by VUV excitation is complex and is often governed by multiple electronic potential energy surfaces, the unraveling of the complex dissociation dynamics requires state-to-state cross section measurements. The newly constructed Dalian Coherent Light Source (DCLS), which is capable of generating coherent VUV radiation with unprecedented brightness in the range of 50-150 nm, promises to propel the photodissociation experiment to the next level.     

Photoprocesses induced by short-wavelength excimer laser radiation in adsorbed halomethane molecules

Photolysis and desorption induced by ArF and KrCl laser irradiation of CH₂I₂, CH₃I, CCl₄, CHCl₃, CH₂Cl₂, CF₂Cl₂, and CHF₂Cl molecules adsorbed on fused silica were studied. One-photon fragmentation, as well as nonlinear dissociation and desorption processes, were observed. It was found that three-step fragmentation is a widespread phenomenon for molecules that experience resonant absorption of laser radiation. To explain the phenomenon, a mechanism based on stimulated emission was proposed. The nonlinear character of desorption is associated with redistribution of absorbed energy in adsorbed molecules and its transfer to the substrate.     

Formation of selectivity in molecular isomerization by the IR multiphoton excitation

The formation mechanism of the selectivity of IR laser isomerization induced by vibrational multiphoton excitation is considered. The effective and highly selective isomerization of perfluorodimethyl ketene (CF₃)₂C=C=O into perfluoromethacrylic acid fluoride F₂C=C(CF₃)COF and perfluorocyclobutene into perfluorobutadiene upon pulse irradiation with a CO₂ laser and its second harmonic was performed. The conversion of (CF₃)₂C=C=O into F₂C=C(CF₃)COF was higher than 99%. A record-breaking conversion of 99.8% of the parent substance into the isomer was achieved in the case of perfluorocyclobutene isomerization into perfluorobutadiene. It was shown that the high selectivity of the laser-induced chemical reactions is mainly associated with the different levels of the vibrational excitation of the parent molecules and their isomers. The latter is due to the difference in the IR absorption spectra of different isomers, which allows for the excitation of the necessary component with a high selectivity.     

Molecular beams of CF3Br probed by spontaneous raman effect and excited with a CO2 laser

The state population of CF₃Br is found to be entirely non-thermal under certain molecular beam conditions; the various vibrational modes show distributions which can be described using mode-temperatures differing by as much as a factor of 1.7. Considerable vibrational excitation (ν₁, ν₂ + ν₃) was produced with a focused cw CO₂ laser. A structured excitation spectrum was observed.     

Excited state electron transfer from photoexcited fac-CIRe(CO)3(2,2′-bipyridine) to N,N′-dimethyl-4,4′-bipyridinium detected by transient

Pulsed laser excitation of fac-CIRe(CO)₃(bpy) in CH₃CN containing N,N′-dimethyl-4,4′-bipyridinium, MV²⁺, results in excited state electron transfer to form MV⁺ that can be detected by the Raman scattering from the trailing portion of the 18 ns 354.7 nm pulse from the frequency-tripled Nd : YAG laser excitation source. Essentially the same results are found for the pulsed excitation of Ru(bpy)₃²⁺ in H₂O containing MV²⁺.     

Mechanisms of the IR laser initiation of combustion in a supersonic H2/O3/O2 flow

The ignition of a supersonic H₂/O₃/O₂ flow by the λ_I = 9.7 μm laser excitation of asymmetric vibrations in O₃ molecules is considered. The O₃ molecules vibrationally excited by laser radiation dissociate more readily to generate active O atoms, thus accelerating the ignition/combustion process. Even at a low input of radiation energy (～7 × 10^?4 J/cm³), combustion in the supersonic flow can be initiated at a short distance (<1 m) from the irradiation zone and at a low gas temperature (～300 K).     

Photolysis of molecules by an infrared high-power laser pulse

We report the spectra of visible luminescence which accompanies the infrared photolysis of molecules by an intense CO₂ laser pulse. The molecules studied were C₂H₄, C₂F₃Cl, CH₃OH, CH₃NO₂, CH₃CN, CF₂Cl₂ and BCl₃. A gated system recorded the signal during the first 100–200 ns after the laser pulse, i.e., during the collisionless dissociation phase. The dissociated products were identified. The mechanism of the infrared photolyses is discussed and compared with UV photolyses.     

Evolution of Laser Plume upon Graphite Ablation in Vacuum and Nitrogen

The propagation of carbon laser plasma during its expansion in vacuum and in a nitrogen atmosphere was studied with the use of an Nd : YAG laser (λ = 532 nm, τ = 15 ns). The presence of C₂, C₃, and CN molecules was detected in emission spectra, and the space and time characteristics of their propagation were determined. The vibrational temperatures of C₂ and CN molecules were calculated.     

Laser isotope separation and the multiphoton decomposition of methanol using a pulsed HF or DF laser

The focussed beam from a single line [P₁(6)] pulsed HF laser has been used to decompose CH₃OH (for pressures between 0.169 and 14.95 kPa) and isotopic mixtures of methanol. The normalized yields (number of product molecules Pulse/P_METHANOL) of the non-condensable products H₂, CO and CH₄ increased linearly with pressure (for the range ≈ 1 - 7 kPa). For sufficiently low pressures, selective excitation of one component of an isotopic mixture gives an isotopically enriched product. For example, selective excitation of CH₃OH in equimolar of mixture CH₃OH/CH₃OD at a total pressure of 269 Pa gives hydrogen which is enriched 60-fold in H versus D. The degree of isotopic enrichment decreases with increasing mixture pressure. The efficiency of conversion of photon energy to reaction product has been observed to increase linearly with pressure. Decomposition studies have been performed in the presence of additives. These imply that the decomposition of methanol to H₂ involves mainly molecular rather than free radical steps.     

Efficient Preparation of D2 Molecules in v=2 by Stimulated Raman Pumping

We report the preparation of D₂ molecules in v=2 level in molecular beam condition. A single longitudinal mode laser system was used for excitation of D₂ from (v=0, j=0) to (v=2, j=0) with the scheme of stimulated Raman pumping. An excitation efficiency of 25.2% has been achieved, which was determined by the scheme of resonance-enhanced multiphoton ionization. Dependence of relative excitation efficiency on laser energy has been measured. We found that the increasing rate of excitation efficiency became slower as pulse energy of Stokes laser increase, while the excitation efficiency still increases approximately linearly with pump pulse energies up to 60 mJ. The spectral line shapes of Raman transition was also measured at different laser energies and considerable dynamical Stark effect was observed. A single peak was found on the three dimension surface of relative excitation efficiency, indicating the process occurred in the present study is a process of stimulated Raman pumping instead of stimulated adiabatic Raman passage.     

Dissociation of Physisorbed Halomethane Molecules by KrF Excimer Laser Radiation

The competitive processes of photolysis and desorption of CF₂Cl₂, CCl₄, CHF₂Cl, CHCl₃, CH₂Cl₂, CH₂I₂, and CH₃I halomethane molecules physisorbed on fused silica were experimentally studied. These processes were induced by the absorption of high-intensity UV radiation from a KrF excimer laser by these molecules. It was shown that a common feature in the behavior of all of these compounds is nonlinear dependence of desorption on radiation intensity, typical nonlinearity indices being very high: n = 5–7. One-photon dissociation was carried out in the adsorbed state of CCl₄, CH₂I₂, and CH₃I molecules absorbing radiation with a wavelength of 248 nm and in a gas phase. The photofragments are characterized by kinetic energies lower than 0.2 eV. The multiphoton fragmentation of the adsorbed molecules was observed and discussed.     

Vibrational relaxation and intermediate strong coupling

In this paper the effect of collisions with inert gas molecules upon the intermediate strong coupling situation, found in the fluorescence of isolated pyrene molecules after pulse laser excitation in the S₀ → S₃ absorption band, is studied.     

Many-body perturbation theory,coupled-pair many-electron theory,and the importance of quadruple excitations for the correlation problem

Many-body (diagrammatic) perturbation theory (MBPT ), coupled-pair many-electron theory (CPMET ), and configuration interaction (CI ) are investigated with particular emphasis on the importance of quadruple excitations in correlation theories. These different methods are used to obtain single, double, and quadruple excitation contributions to the correlation energy for a series of molecules including CO₂, HCN, N₂, CO, BH₃, and NH₃. It is demonstrated that the sum of double and quadruple excitation diagrams through fourth-order perturbation theory is usually quite close to the CPMET result for these molecules at equilibrium geometries. The superior reliability of the CPMET model as a function of internuclear separation is illustrated by studying the ¹∑ potential curve of Be₂. This molecule violates the assumption common to nondegenerate perturbation theory that only a single reference function is important and this causes improper behavior of the potential curve as a function of R. This is resolved once the quadruple excitation terms are fully included by CPMET .     

Collision-assisted multiple step excitation of CH3F by irradiation with a TEA CO2 laser

By irradiating a mixture of CH₃F highly diluted in Ar with a TEA CO₂ laser, the v₃ overtone emission rises with a rate much larger than the “up-the-ladder” V-V pumping rate indicating that the CH₃F is excited up to the 3v₃ level almost instantly. This occurs via multiple step excitation assisted by rotational transitions in collisions.