Electronic spectra of uracil in a supersonic jet

The fluorescence excitation and dispersed fluorescence spectra of uracil in a supersonic jet have been observed. The n,π¹ states of 2,4-diketo tautomer and enol-keto tautomer are found. The spectral analyses show an out-of-plane deformation of the molecules in the n,π¹ state. The coexistence of diketo and enol-keto tautomer in the vapor is discussed.     

Electronic spectroscopy and photoisomerization of trans-urocanic acid in a supersonic jet.

trans-Urocanic acid (trans-UA), a component of the epidermal layer of skin, exhibits wavelength-dependent photochemistry. The quantum efficiency of isomerization to cis-UA is greatest when the molecule is excited on the long wavelength tail of its absorption profile in solution (300-320 nm). However, exciting the molecule where it absorbs UV light most efficiently (260-285 nm) causes almost no isomerization. We have used fluorescence excitation and dispersed emission methods in a supersonic jet to investigate the electronic states involved in this complex and interesting photochemistry. Three distinct regions are present in the excitation spectrum. Region I, which is below the isomerization barrier, contains sharp, well-resolved peaks that upon excitation emit from the S(1) state of trans-UA. Region II exhibits peaks that increase in broadness and decrease in intensity with increasing excitation energy. Upon excitation these peaks produce dual emission from the S(1) states of both trans- and cis-UA. The trans to cis isomerization barrier is estimated to be 1400 cm(-1). Region III exhibits excitation to the S(2) electronic state and has a broad structure that spans 3000 cm(-1) and occurs 4000 cm(-1) above S(1). S(2) excitation results in essentially no trans to cis isomerization.     

Equations for translational relaxation in a steady supersonic jet of a mixture of monoatomic gases

On the basis of the Boltzmann kinetic equation with the use of the ellipsoidal distribution function a system of equations that takes the two-dimensional flow near a nozzle into account was derived for the parameters of a supersonic steady jet expanding into a vacuum.     

Electronic spectra of hydrogen-bonded 2-fluoropyridine clusters with water in a supersonic free jet

Fluorescence excitation, multiphoton ionization, and dispersed fluorescence spectra of bare and hydrogen-bonded 2-fluoropyridine with water were measured in a supersonic free jet. For bare 2-fluoropyridine, fluorescence quantum yield decrease in the higher vibronic levels was observed even under collision-free conditions. The inter-system crossing channel was probed experimentally by two color R2PI and found to be negligible. The non-radiative relaxation process of 2-fluoropyridine is mainly governed by the relaxation to the electronic ground state. Electronic spectra of 2-fluoropyridine-(water)(n) (n=1 approximately 3) were also obtained. The hydrogen bond formation with water increases the quantum yield in the higher vibronic levels. Rather low frequency vibrations were observed in the hole burning spectrum of bare 2-fluoropyridine; however, these vibronic bands disappeared with the hydrogen bond formation with water. The appearance of low frequency vibronic bands observed for bare 2-fluoropyridine is ascribed to the existence of closely lying (n,pi) state.     

The electronic relaxation of biacetyl in the vapor phase

The emissions of biacetyl after pulsed dye-laser excitation were studied at pressures down to 0.05 mtorr. At all energies the time-resolved fluorescence was composed of a nanosecond and a microsecond component. At “zero” pressure the long lived phosphorescence was absent while the “hot” phosphorescence has the same time characteristics as the slow fluorescence. By increasing the pressure the slow fluorescence was quenched while the milisecond phosphorescence was induced. We determined the low-pressure emission characteristics and the pressure effects as a function of excitation energy.From our data we obtained the parameters describing the intermediate type singlet-triplet coupling, the radiative and non-radiative relaxation rates from the singlet and triplet levels and the cross sections for the slow fluorescence quenching, all as a function of energy. Strong evidence is obtained for the participation of rotational states in the intra-molecular relaxation. The important difference between the situation where the singlet levels are isolated (low energy) and where the singlet level widths overlap (at higher energies) is demonstrated. In the former situation very large fluorescence quenching cross sectios were found. It is further shown that for high energies at least two effective collisions are needed to obtain a thermalized triplet; the mean energy removed per effective collision is 2200 cm^?1.     

Reactions of hydrocarbons in a supersonic vacuum plasma jet

The plasma plume of a hydrogen plasma jet used for diamond synthesis is analyzed by a Pitot tube and by mass spectrometry. In the investigated pressure range of 2–10 mbar, supersonic gas velocities with Mach numbers of up to 2 were observed, which decreased with increasing pressure and increasing distance from the nozzle. The injection of the carbon-containing species either at the exit of the jet nozzle or simply into the background gas of the reaction chamber confirmed the importance of recirculation of background gas into the plasma plume. In the case of background injection the rise of the total carbon content in the plume with increasing distance from the nozzle is much slower than in the case of nozzle injection. The results of a numerical model of the hydrocarbon gas-phase reactions in the jet are presented. The model considers the entrainment of background gas into the plasma plume. Two domains along the jet axis can be distinguished. The first one in the vicinity of the nozzle is dominated by methyl radicals, the second one by atomic carbon. Increase of the hydrogen dissociation level results in the broadening of the atomic carbon domain and the rise of C₂ far from the nozzle. Background injection of CH₄ leads to lower total carbon content in the plume but has little effect on the species distribution along the jet axis.     

The vibrational relaxation of I2 by H2 in a supersonic jet

The vibrational relaxation of I₂ by H₂ has been studied in a supersonic free jet. It was observed that the addition of 5% H₂ to the helium carrier gas greatly reduces the concentration of X ¹Σ⁺_g(ν″ = 1) I₂ in the jet as compared to the concentration in a pure helium carrier. From this observation we have determined that the average vibrational relaxation cross sections of H₂ is 7.1 times as large as that of helium. Since the average vibrational relaxation cross section of deuterium is at least as large as that of hydrogen, the mechanism responsible for this phenomenon appears not to be dominated by mass effects.     

Two-color multiphoton ionization of diazabicyclooctane in a supersonic free jet

Two-color multiphoton ionization (MPI) spectroscopy has been applied for diazabicyclooctane (DABCO) in a supersonic free jet. The MPI spectra due to transitions from the various vibronic levels of the S₁ (3s Rydberg) state which were excited by the first laser revealed the high Rydberg states above the adiabatic ionization potential. The ionization process and the vibrational potential of the ion are discussed.     

Vibrational relaxation of biacetyl studied by a visible—infrared double resonance technique

The vibrational relaxation of biacetyl following excitation by a highly intense laser pulse at 10.6 micron is studied. The changes in vibrational population are monitored by visible absorption from the ground electronic state to the excited n-π* state. The results indicate that thermal disequilibrium persists for at least 10^?5 seconds after the laser pulse at pressures in the range of 1–40 torr.The implications of these results for the use of high-power infrared lasers in inducing selective chemical reactions are briefly discussed.     

Vibrational relaxation of triplet chlorotoulenes studied by photosensitized phosphorescence of biacetyl

The photosensitized phosphorescence of biacetyl by chlorotoulene vapors has been investigated under stationary conditions. Quantum yields of stable triplet formation for chlorotoulenes increased with increasing foreign gas (ethane) pressure, suggesting that vibrational relaxation of initially formed triplet levels is competitive with photodissociation of the CCl bond. Foreign-gas pressure effects on the triplet formation yield by the 0-0 excitation has lead to photodecomposition rate constants of chlorotoluenes ( ≈ 10² s^?1). Step-ladder collisional relaxation processes have been postulated for the excitation at shorter wavelengths.     

Diagnostics of a supersonic jet in a high-pressure background by infrared absorption

Using a diode laser we have measured infrared absorption of molecular beams of ammonia and freon-12, expanding in a volume evacuated at relatively high pressures (10^?4-10^?2 bar). The effects of the background gas seem to be negligible. We observed large deviations from a Boltzman rotational energy distribution, as well as the formation of molecular complexes. Rotational temperatures as low as 10 K have been obtained in a beam of freon-12 mixed with helium.     

Photophysics and photochemical dynamics of methylanisole molecules in a supersonic jet

The fluorescence excitation, dispersed fluorescence and hole burning spectra, and fluorescence lifetimes of jet-cooled o-, m-, and p-methylanisoles (MA) were measured. The low-frequency ring methyl internal rotational bands observed for their S₀ and S₁ states were assigned. In the case of m-MA, the rotational isomers of cis and trans conformers, which arise from the orientation of the OCH₃ group with respect to the CH₃ group, were assigned by hole-burning spectroscopy. The observed level energies and relative intensities of the methyl internal rotation were reproduced by a calculation using a free rotor basis set. Furthermore, their potentials in the S₀ and the S₁ states were determined. The potential barrier heights for the S₀ states of m- and p-MA were quite low, suggesting that the methyl groups are freely rotating, while changing from S₀ to S₁ states, the potential barrier height increases. The potential barrier heights of o-MA drastically decreased in going from S₀ to S₁ states. The decrease would be due to the hydrogen bonding between O atom and one H atom of the methyl group. The torsional bands of the methoxy group (–OCH₃) were also observed for p- and o-MA. The –OCH₃ modes are found to couple with the level of the e species for the methyl internal rotation.Fluorescence lifetimes (τ_f) of the methyl internal rotational bands in the S₁ states of o-, m-, and p-MA were measured in order to investigate the photochemical dynamics. The values of the nonradiative rate constant (k_nr) were estimated from the τ_f values and Franck–Condon factors. The k_nr values drastically increased with the excitation of methyl internal rotation. Accordingly, the methyl internal rotation should enhance the nonradiative process, presumably intersystem crossing (ISC). The enhancement should be caused by the increase of the state density (ρ) effectively coupled with triplet manifolds. The drastic increase in the ρ value should be caused by level mixing. In addition, the methyl internal rotational motion may enhance the increase of the coupling matrix elements through the vibronic coupling between the excited singlet states. The remarkable rotational quantum species dependence on the ISC rate constant (k_ISC) value clearly appeared in m-MA. The dependence should result from the difference of the ρ value between a₁ and e species, since the e species are doubly degenerate. The species dependence was apparently related to the potential barrier height, suggesting that the large barrier height should have an influence on the ρ value of the triplet states.     

Rotational and vibrational temperatures of naphthalene in a naphthalene-argon supersonic jet

In the $\text{A}$¹B_2u-$\text{X}$¹A_g system of naphthalene in a supersonic jet, rotational contour calculations show rotational temperatures of 2–60 K for argon carrier gas pressures of 1520-120 Torr. The b_1u vibration v₂₄ shows a high vibrational temperature which corresponds to the seeding temperature for pressures <400 Torr.     

Ring inversion of dihydronaphthalene by laser-induced fluorescence in a supersonic jet

The laser-induced fluorescence excitation (FE) and the single vibronic level fluorescence (SVL) emission spectra of 1,2-dihydronaphthalene have been studied in a pulsed supersonic jet. The 128 cm⁻¹ progression in FE and the 140 cm⁻¹ progression in SVL have been assigned to the low-frequency large-amplitude torsional motion between the two symmetry-related non-planar conformations of the excited and the ground states, respectively. Comparison of frequencies and intensifies with corresponding values calculated from a model double-well potential leads to a barrier height of 640 cm⁻¹ for the excited state and greater than 1000 cm⁻¹ for the ground state.     

Chromatographic interfaces for supersonic jet spectroscopy

Until recently, applications of supersonic jet spectroscopy to chemical analysis have been plagued by relatively poor detection limits and the lack of suitable interfaces to standard chromatographic techniques. Supersonic jet nozzles based upon new technologies have been developed which permit low picogram detection limits to be obtained and provide convenient interfaces for capillary gas chromatography. Extension of these methods to liquid and supercritical fluid chromatography may also be possible.     

Nonequilibrium effects in a steady supersonic jet of a mixture of monatomic gases

On the basis of the moment equation system for parameters of a steady supersonic jet of a mixture of monatomic gases, the analysis of nonequilibrium effects as a velocity slip and temperature difference of components was carried out in hypersonic and spherically symmetric approximations. The limiting values of velocity slips and kinetic temperatures of mixture components depending on the source jet conditions and interaction potential were obtained.     

Ion dip spectra toluene and aniline in a supersonic free jet

The ion dip spectra toluene and aniline were measured using a supersonic free jet. It was demonstrated that an ion dip spectrum gives a ratio of the transition probablity from a ground state to a resonant intermediate state and that from the intermediate state to the ionization continuum.     

Femtosecond rotational Raman coherence spectroscopy of cyclohexane in a pulsed supersonic jet

We combine the technique of femtosecond degenerate four-wave mixing (fs-DFWM) with a high repetition-rate pulsed supersonic jet source to obtain the rotational coherence spectrum (RCS) of cold cyclohexane (C(6)H(12)) with high signal/noise ratio. In the jet expansion, the near-parallel flow pattern combined with rapid translational cooling effectively eliminate dephasing collisions, giving near-constant RCS signal intensities over time delays up to 5 ns. The vibrational cooling in the jet eliminates the thermally populated vibrations that complicate the RCS coherences of cyclohexane at room temperature [Brügger, G.; et al. J. Phys. Chem. A 2011, 115, 9567]. The rotational cooling reduces the high-J rotational-state population, yielding the most accurate ground-state rotational constant to date, B(0) = 4305.859(9) MHz. Based on this B(0), a reanalysis of previous room-temperature gas-cell RCS measurements of cyclohexane gives improved vibration-rotation interaction constants for the ν(32), ν(6), ν(16), and ν(24) vibrational states. Combining the experimental B(0)(C(6)H(12)) with CCSD(T) calculations yields a very accurate semiexperimental equilibrium structure of the chair isomer of cyclohexane.     

Intramolecular electronic energy transfer of bichromophoric molecules in a supersonic free jet

The intramolecular electronic energy transfer between two chromophores separated by a methylene chain has been observed in an isolated molecule. The mo     

Reinterpretation of the fluorescence excitation spectrum of hexafluorobiacetyl in a supersonic jet

The results of the analysis of the fluorescence excitation spectrum of trans-hexafluorobiacetyl published earlier are revised. New values of torsion frequencies and potential barriers of internal rotation of the CF₃ group in the ground and excited states of the molecule are obtained. A procedure for calculating the probabilities of torsional vibronic transitions of molecules is described. Translated fromZhurnal Struktumoi Khirnii, Vol. 38, No. 2, pp. 293–302, March–April, 1997.