Laser studies of pyridylindoles in supersonic jets

One- and two-photon spectroscopy has been used to study jet-cooled 2-(2′-pyridyl)indole (PyIn-0) and 3,3′-dimethylene-2-(2′-pyridyl)indole (PyIn-2). The laser-induced fluorescence and resonance-enhanced photoionisation spectra of PyIn-2 are congested due to the flexibility of the ethylene chain. Single vibronic level fluorescence spectra allow identification of equivalent modes in the ground and electronically excited states. Deuteration at nitrogen reveals a 4 cm⁻¹ red shift of the origin. Ab initio calculations were conducted at the TD B3LYP/6-31G(d,p) level of theory. We were able to reproduce the energies of pure electronic transitions both in PyIn-0 and PyIn-2. The properties of pyridylindoles in supersonic jets are compared with those for indole and 2-phenylindole.     

Photoisomerization dynamics of alkyl-substituted stilbenes in supersonic jets

We report time-resolved fluorescence decay lifetimes from photoselected states of jet-cooled 4-ethyl-trans-stilbene (4ETS) and 4-n-propyl-trans-stilbene (4PTS) in the energy range E_v = 0–2800 cm⁻¹ above the S₁ origin. Alkyl substitution enhances the non-radiative decay rates. These data can be fitted by the statistical RRKM theory in conjunction with the reduction of the photoisomerization threshold from E_t = 1300±50 cm⁻¹ for trans-stilbene to E_t = 1100±100cm⁻¹ for 4ETS, and E_t = 1000±100 cm⁻¹ for 4PTS.     

Precision velocity measurements of pulsed supersonic jets

We introduce a straightforward experimental approach for determining the mean flow velocity of a supersonic jet with very high precision. While time measurements easily can achieve accuracies of Δt/t ≤ 10(-4), typically the absolute flight distances are much less well-defined. This causes significantly increased errors in calculations of the mean flow velocity and mean kinetic energy. The basic concept to improve on this situation is changing the flight distance in vacuo by precisely defined increments employing a linear translation stage. We demonstrate the performance of this method with a flight path that can be varied by approximately 15% with a tolerance of setting of 50 μm. In doing so, an unprecedented accurate value for the mean flow velocity of Δv/ < 3 × 10(-4) has been obtained without prior knowledge of the total distance. This very high precision in source pressure, temperature, and particle speed facilitates an improved energetic analysis of condensation processes in supersonic jet expansions. The technique is also of broad interest to other fields employing the strong adiabatic cooling of supersonic beams, in particular, molecular spectroscopy. In the presented case study, a thorough analysis of arrival time spectra of neutral helium implies cluster formation even at elevated temperatures.     

Infrared spectroscopy of hydrogen-bonded 2-fluoropyridine-water clusters in supersonic jets

The hydrogen-bonded clusters of 2-fluoropyridine with water were studied experimentally in a supersonic free jet and analyzed with molecular orbital calculations. The IR spectra of 2-fluoropyridine-(H2O)(n) (n = 1 to 3) clusters were observed with a fluorescence detected infrared depletion (FDIR) technique in the OH and CH stretching vibrational regions. The frequencies of OH stretching vibrations show that water molecules bond to the nitrogen atom of 2-fluoropyridine in the clusters. The hydrogen-bond formation between aromatic CH and O was evidenced in the 1:2 and 1:3 clusters from the experimental and calculated results. The overtone vibrations of the OH bending mode in hydrogen-bonded water molecules appear in the IR spectra, and these frequencies become higher with the increase of the number of water molecules in the clusters. The band structure of the IR spectra in the CH stretching region changes depending on the number of coordinating water molecules.     

Fluorescence of dissociating fragments from supersonic jet-electron collisions

Supersonically cooled jets of nitrogen, methane, ethane, cyclopropane, and azomethane are crossed with collimated streams of electrons. The CH (B²Σ^? → X²Π) spectra resulting from the electron-induced dissociation of CH₄, C₂H₆, and CH₂)₃ can be fit with rotation temperatures between 4000 and 6000 K for an electron energy of 100 eV. Flourescence spectra of N₂⁺ (B²Σ_w⁺ → X²Π) from the dissociative ionization of azomethane yield a rotational temperature of =8×10³ K; from ionization of molecular nitrogen the rotational temperature of B²Σ_w⁺ N₂⁺ is 45 K. Mechanisms for these various processes are discussed.     

Conformational interconversions in supersonic jets: Matrix IR spectroscopy and model calculations

Terminal conformer populations in supersonic molecular beams have been measured by use of matrix IR spectroscopy. the experimental technique is based on trapping of the beam molecules in to a cryogenic matrix. The ratio of conformational isomers is determined by comparing intensity ratios of infrared absorption bands with those found in analogous experiments with thermal effusive molecular beams. supersonic beams of pure 1,2-difluoroethane a considerable depopulation fo the less stable trans conformer is found, the lowest terminal conformational temperature reached being T_c^t8 = 207 (3) K. In seeded argon beams the cooling of the conformational distribution was found to be weaker. In supersonic beams of 1,2-dichloroethane and of methyl nitrite no significant conformational cooling was found. The experimental results are discussed in terms of a kinetic model of conformational interconversion in the flow field of a continuum free jet. The calculation indicate that conformational cooling by supersonic expansion may be expected only for molecules with a low energy barrier to internal rotation.     

Relaxation in pure and seeded supersonic jets of SF6

Nozzle expansion of pure SF₆ and SF₆ seeded in carrier gases e.g. Ar, He and N₂, is used to study relaxation of the various degrees of freedom. The diode laser absorption spectrum of the Q branch of the ν₃ mode of SF₆ is used to derive the rotational temperature (T_rot). Time-of-flight studies determine the translational temperature (T_tg) while the vibrational temperature (T_vib) is estimated from the energy balance equation. Rotational and translational temperatures follow each other closely while the vibrational temperature lags behind considerably. The effective specific heat ratio (γ) and the collisional effectiveness parameter (e) for SF₆ are determined from these measurements. Seeded-beam data prove argon to be a better refrigerant than He or N₂. Cooling in seeded beams at moderate stagnation pressures is not substantially enhanced even at very high dilution.     

Cluster size effects in vuv radiation spectra of argon and krypton supersonic jets

The relation between VUV radiation spectra of argon and krypton supersonic jets excited by an electron beam and the mean size of clusters in a jet is studied. For each atomic and molecular emission of argon in the 104–120 nm range and of krypton from 116 to 130 nm, the maximum intensity is found to occur when electrons collide with clusters of a definite mean size. It is found that self-localized excitons of atomic type or diatomic excimer molecular type with a small binding energy (several hundredths of an eV) are mainly formed in outer icosahedral shells of argon or krypton clusters and those of diatomic excimer molecular type with a large binding energy (several tenths of an eV) arise principally in inner shells.     

Dynamics of radiationless processes studied in pulsed supersonic free jets: Some naphthalene lifetimes

A pulsed supersonic free jet is used to prepare a low temperature sample of naphthalene vapor. Single vibronic level fluorescence lifetimes determined therein differ significantly from lifetimes previously obtained at room temperature.     

Fluorescence spectra of fluorobenzene cations excited in a supersonic molecular beam

Fluorescence spectra of several fluorobenzene cations in the gas phase have been observed following electron impact on a supersonic beam of the neutral molecule. The very low rotational temperature of the beam is not disturbed by the ionisation process, so the different vibronic states of the cation ar produced rotationally cold; the fluorescence spectra are therefore very simple as every vibronic band has been condensed into a very few rotational com Vibrational frequencies obtained from the analyses agree excellently with values from other techniques. The effects of Jahn—Teller distortion are cle in the spectra of C₆F₆⁺ and 1,3,5-C₆F₃H₃⁺.     

On collisionally enhanced laser pumping of supersonic jets of SF6

Electron-diffraction analyses of SF₆ molecules in a microjet irradiated by a cw infrared laser are reported. Excitations of several photons per molecule are achieved in the collisional region extending for the order of a nozzle diameter before molecules become cold and dilute. A model accounting for results is proposed.     

Structures of methyl halide dimers in supersonic jets by matrix-isolation infrared spectroscopy and quantum chemical calculations

The CH₃Cl and CH₃Br dimers produced by supersonic-jet expansion were directly deposited on a cold plate using a standard matrix-isolation technique. Dependence of the relative intensities of the observed infrared bands on the stagnation pressure was used to assign the dimer bands appearing near the monomer bands. By a comparison of the wavenumber shifts from the monomer bands with the corresponding values obtained by quantum chemical calculations, DFT/B3LYP/6-311++G(3pd,3df) and MP2/LanL2DZ+fdp, the structures of CH₃Cl and CH₃Br dimers were determined to be a head-to-tail isomer, which is common to the CH₃F and CH₃I dimers determined previously by the same method. The remaining dimer bands, which could not be assigned to the head-to-tail isomer, were tentatively assigned to a head-to-head isomer in analogy with CH₃I dimer.     

Laser-induced fluorescence and hole-burning spectra of functional dyes in supersonic jets: spectra of a merocyanine dye

In LIF (laser-induced fluorescence) excitation and hole-burning spectra of a merocyanine dye, EtMD, in a supersonic jet, bands due to one of the four possible isomers were observed. Only lower-frequency vibronic bands were observed in the LIF-excitation spectra, while broad and deep dips were observed in the higher energy region of the hole-burning spectra where no LIF bands were observed. The LIF bands were found to consist of two transition systems, which are presumably due to the molecules in the ground and low-lying excited vibrational states. Possible relaxation processes of EtMD in the excited electronic state were discussed.     

Tunable diode laser spectrometer for pulsed supersonic jets: application to weakly-bound complexes and clusters

The design and operation of an apparatus for studying infrared spectra of weakly-bound complexes is described in detail. A pulsed supersonic jet expansion is probed using a tunable Pb-salt diode laser spectrometer operated in a rapid-scan mode. The jet may be fitted with either pinhole or slit shaped nozzles, the former giving lower effective rotational temperatures, and the latter giving sharper spectral lines. Notable features of the apparatus include use of a toroidal multi-pass mirror system to give over 100 passes of the laser through the supersonic jet, use of the normal laser controller for laser sweeping during both setup and data acquisition, and use of a simple semi-automated wavenumber calibration procedure. Performance of the apparatus is illustrated with observed spectra of the van der Waals complex He-OCS, and the seeded helium clusters He(N)-OCS and He(N)-CO.     

Photolysis of glyoxal in air

The photolysis of glyoxal in synthetic air was investigated in a quartz cell at 298 K using three types of UV sources (TL/12 lamps (275–380 nm), TL/03 lamps (390–470 nm) and mercury lamps (254 nm)) and products were identified and quantitatively analyzed using long-path FTIR spectroscopy. For all light sources, the observed products were CO, HCHO and HCOOH. Absolute quantum yields were determined using Cl₂ and Br₂ as actinometers. Photolysis in the first absorption band of glyoxal, using TL/12 lamps, provided an overall quantum yield of Φ_T = 0.97 ± 0.05, independent of total pressure ranging from 100 to 700 Torr air. The absolute quantum yields obtained with the TL/03 lamps, covering the second absorption band of glyoxal, showed dependency on total pressure, ranging from Φ_T = 0.12 at 100 Torr to Φ_T = 0.042 at 700 Torr, which can be expressed as a Stern–Volmer-type equation 1/Φ_T = (6.80 + 251.8) × 10⁻⁴ × P (Torr).By combining the product yields with literature data, we deduced the detailed picture of glyoxal photolysis, including the dependency of the quantum yield of each particular channel: CHOCHO + hν → 2HCO (Φ₁); CHOCHO + hν → H₂ + 2CO (Φ₂); CHOCHO + hν → H₂CO + CO (Φ₃) on the applied wavelength. The product quantum yields indicate that dissociation into two HCO radicals is the most important pathway under atmospheric conditions. The mean photolysis rate was measured under solar radiation in the EUPHORE outdoor chamber to be J_obs = 1.04 ± 0.10 × 10⁻⁴ s⁻¹, corresponding to a mean effective quantum yield ϕ_eff = 0.035 ± 0.007. Although glyoxal has a very low effective quantum yield, photolysis remains an important removal path in the atmosphere.     

Structural preferences, argon nanocoating, and dimerization of n-alkanols as revealed by OH stretching spectroscopy in supersonic jets

n-Alkanols can occur in a multitude of energetically competitive conformational states. Using the OH stretching vibration as an infrared and Raman spectroscopic sensor in supersonic jet expansions, the torsional preferences around the Calpha-O and Cbeta-Calpha bonds are probed for n-propanol through n-hexanol. Raman detection is more powerful for isolated monomers, whereas IR spectroscopy is more sensitive for molecular complexes. The subtle IR vibrational shift induced by the nanocoating of n-alcohols with Ar atoms is shown to alternate with chain length. A large number of alcohol dimer absorptions is observed and subjected to collisional relaxation and nanocoating conditions. Essential features of the dimer spectra are modeled successfully by a simple force field approach. Exploratory quantum chemical calculations up to the MP2/aug-cc-pvqz level encourage a rigorous theoretical study of the subtle conformational aspects in monomers and possibly also in dimers of linear alcohols.     

The vacuum-UV spectra of supersonic jets of ArKrXe mixtures excited by an electron beam

The vacuum-UV spectra of supersonic jets of ArKrXe mixtures excited by an electronic beam were investigated. At small concentrations of Kr and Xe admixtures to the Ar jet, intensive continuous spectra of Kr and Xe were observed. Continuous spectra with intensity maxima at 1350 Å and 1540 Å were also recorded and interpreted as the spectra of the heteroatomic molecules ArKr and KrXe.     

Laser-induced fluorescence spectra of Ne- and Ar -aniline van der waals complexes in supersonic jets. Analysis of rotational structures

Laser-induced fluorescence spectra of Ne- and Ar -aniline complexes were observed with a resolution of 0.04 cm^?1. By a simulation analysis of the rotational structures, the distances between the rare-gas atom and the aromatic ring of aniline were found to be 3.35 ± 0.04 and 3.50 ± 0.04 A in the X?¹A₁ state of the Ne and Ar complexes, respectively, and about 0.05 A shorter when aniline was excited to the ùB₂ state.     

Cis-trans isomerization of glyoxal

The CNDO/2 method with the original parameter set predicts the conformation with the mutually perpendicular CH=O groups to be the most stable isomer of glyoxal; the cis form is favoured over the trans form. The order of stabilities is not changed upon the full optimization of coordinates of the isomers. Calculations fail to reproduce the observed order of isomer stabilities unless allowance is made for limited configuration interaction with the lowest ππ* doubly excited state and for geometry optimization. Doubly excited configurations of the ππ* and σσ* types have negligible effect on the energies of isomers.     

Long-lived fluorescence decay components in glyoxal

Long-lived components in glyoxal are considered to have been observed in the recent sensitized phosphorescence experiments reported by Ito and co-workers; this interpretation explains the apparent collision-free “intersystem crossing”, which is inconsistent with the accepted idea that glyoxal behaves according to the weakly coupled small-molecule limit. Mode selectivity as well as rovibronic level selectivity is also interpreted.