John G. Philis

Absolute vacuum ultraviolet absorption spectra of some gaseous azabenzenes

Abstract Absolute extinction coefficient and oscillator strength of pyridine, pyrimidine, pyrazine, and s-triazine were recorded with moderate resolution in the region of 3.5–9.5 eV. Analogous to 1 A 1 g → 1 B 2 u , 1B1u, 1 E 1 u π → π ∗ transitions of benzene, several n → π ∗ and Rydberg transitions are presented and discussed.

The extinction coefficient of benzene vapor in the region 4.6 to 36 eV

Abstract Extinction coefficient spectra and oscillator strength values for the benzene transitions in the 4.6 to 9.2 eV region have been measured taking into account the effect of the instrumental bandwidth. By normalizing Koch and Ottos ( 1 ) uncalibrated absorption spectrum at 6 to 36 eV with our spectrum recorded with the same bandpass we provide values of ϵ in the region 4.6 to 36 eV. The dependence of ϵ max on the bandpass is presented and a comparison is made between theoretical band intensities in a progression and band intensities measured experimentally. A total integrated intensity of 0.075 is found for the Rydberg transitions below the first I.P. and a total value of 17.5 is found for the whole region 4.6 to 36 eV.

The methylbenzenes vis-à-vis benzene: Comparison of their spectra in the valence-shell transitions region

Abstract Absolute extinction coefficient and oscillator strength values of nine gaseous benzenes (toluene, o-, m-, p-xylene, 1,2,4-, 1,3,5-trimethylbenzene, 2,3,5,6-tetramethylbenzene, pentamethylbenzene, and hexamethylbenzene) were recorded with moderate resolution in the region of the lower valence-shell electronic transitions (4 to 7 eV). The spectra are very similar to benzene except for a new band that appears in the heavier substituted compounds.

Vibrational assignments in the 3p Rydberg states of acetone

The 2 and 3 photon resonantly enhanced multiphoton ionization spectra of the 3p Rydberg ←X transitions of acetone and acetone‐d6 were remeasured and reanalyzed. Seven of the eight a1 and one b1 vibrational modes were assigned. On the basis of these assigned fundamental frequencies the anomalous isotope shift of the origin of the A1 transition was rationalized, the contribution of the nonbonding electrons to the scissors force constant was demonstrated, and a location for the A1 ππ* valence excited transition was proposed.

The methylbenzenes vis‐a‐vis benzene. Comparison of their spectra in the Rydberg series region

Absolute extinction coefficient spectra of nine gaseous methylbenzenes (toluene, o‐, m‐, and p‐xylene, 1,2,4‐ and 1,3,5‐trimethylbenzene, 2,3,5,6‐tetramethylbenzene, pentamethylbenzene, and hexamethylbenzene) have been recorded with a moderate resolution of 2.5 A in the region of the Rydberg excitations below the first I.P. Term values, quantum defects, and ionization potentials have also been determined for each Rydberg series. Comparison with the benzene spectrum shows the importance of s‐type Rydberg series in the spectra of these compounds.

Ionization/dissociation of thiazole and thiazolidine induced by strong laser fields

Abstract The interaction of thiazole and thiazolidine with a strong (∼ 2×10 16 W/cm 2 ) fs laser field has been studied at λ=790 nm by means of time-of-flight (TOF) mass spectrometry. The observed relative abundance of the doubly charged intact parent ion in thiazolidine is higher than that of thiazole, while the laser-molecule coupling strength is found to be much more efficient for the aromatic (thiazole) than the nonaromatic (thiazolidine) molecule. The mass spectra of thiazolidine are attributed to a combination of field ionization with subsequent multiphoton processes. It is also concluded, that direct Coulomb explosion within the transient multiply charged parent ions leads to the production of multiply charged atomic ions.

Spectroscopic VUV and MPI studies of phenylsilane; Fragmentation and production of silicon

Abstract A detailed presentation of the photoabsorption spectrum of phenylsilane vapors, C6H5SiH3, is reported over the region 280-135 nm (4.4–9.2 eV). The three π, π ∗ valence states lying in this region are well resolved and analyzed while the Rydberg transitions are very broad and blended in the absorption “background”. The lowest π, π ∗ valence transition (Sl or Lb) is studied by multiphoton ionization (MPI) as a supplementary technique. In the MPI spectrum a resonance appearing at 487.9 nm is attributed to atomic Si two-photon resonance transition. Silicon is produced by photolysis of phenylsilane with the strong laser green light.

Internal rotation of the methyl group in the S0 and S1 (3s Rydberg) electronic states of N-methylpyrrole

Abstract The one-photon and two-photon REMPI spectra of the ( 1 A 2 ) S 1 ←( 1 A 1 ) S 0 Rydberg transition of jet-cooled N -methylpyrrole have been investigated. The transition is one-photon symmetry forbidden and a number of well-resolved low-frequency bands are present in the one-photon resonant MPI spectrum, on both sides of the absent origin. These spectral features, assigned to pure torsional transitions of the methyl group, gain intensity from an electronic motion–internal rotation coupling mechanism. The analysis afforded the potentials for the internal rotation of the methyl group in both states: V ″ 6 =−45 cm −1 , B ″ =5.37 cm −1 for the ground state and V ′ 6 =17 cm −1 , B ′ =5.24 cm −1 for the S 1 excited state.

Spectroscopy of jet-cooled phenylsilane

Abstract The one-photon and two-photon S 1 ←S 0 spectra of jet-cooled phenylsilane are investigated in order to clear up contradictions in the literature. Both spectra follow the general rules for one-photon and two-photon spectroscopy of substituted benzenes. The electronic origin is unambiguously determined to occur at 37179±1.5 cm −1 .

Multiphoton dissociation/ionization of benzene, pyridine, and diazabenzenes. Detection of an atomic carbon transition

Multiphoton processes in benzene, pyridine, and the diazabenzenes with ∼10 ns, 0.4–2.5 mJ pulsed laser light in the 336–347 nm range, produced resonantly enhanced molecular ions as well as neutral carbon atoms. The carbon atoms were detected by subsequent two‐photon resonant (...2p2 1D2→2hν ...2p3p 1P1) three‐photon ionization at 340.93 nm. MPI resonances due to vibrations of the 3p Rydberg state appeared in the spectra of pyrazine and pyrimidine, as well as a probable member of the first singlet→triplet band in pyrimidine.