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The rotationally resolved triplet-singlet electronic transition of SiF2 is experimentally determined using pyrolysis jet technique. Intensity distribution among the rotational branches in the 0 (sub 0)(super 0) band shows that the intensity of triplet-singlet transition is mainly due to the spin-orbit coupling with excited 1B2 states. Rotational analysis in the 0 (sub 0)(super 0) band shows that the upper state has 3B1 symmetry.

The spectroscopy and excited state dynamics of A 1B1 germylene (GeH2) have been investigated experimentally and theoretically. Jet-cooled laser-induced fluorescence spectra of GeH2 were obtained by subjecting germane (GeH4) to an electric discharge at the exit of a pulsed nozzle. The band origins of ten vibronic transitions were determined, giving values for the upper state fundamentals of ν1=783.0 cm-1 and ν2=1798.4 cm-1. Sufficient numbers of 000 band rovibronic transitions were observed to give the ground and excited state structures as r″=1.591(7) Å, θ″=91.2(8)° and r′=1.553(12) Å, θ′=123.4(19)°. Fluorescence lifetime measurements show that the 00,0 rotational levels decay radiatively; higher J rotational states in the 00 vibronic level decay much faster, due to a heterogeneous predissociation in the excited state. High quality ab initio studies are consistent with a model in which the lower vibronic levels of the A state predissociate through the a 3B1 state to produce Ge(3P)+H2(1Σ+g). The transition state for this process has been located and the barrier to dissociation is 15.2 kcal/mol above the A 1B1 state, so that tunneling through the barrier must occur. Above 4000 cm-1 of vibrational energy in the A state, a breaking off of fluorescence is observed as a second predissociation channel involving GeH2(A 1B1)→Ge(1D)+H2(1Σ+g) becomes accessible. This process is also found to have a barrier, in contrast to previous theoretical studies of SiH2, where the analogous dissociation was predicted to be barrierless. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The jet-cooled laser induced fluorescence excitation spectrum of the A 1A″–X 1A′ band system of HSiF has been observed with the chemical reaction jet technique. Vibrational analysis of the spectrum gave upper state fundamental vibrational frequencies of ν1=1547 cm-1, ν2=558 cm-1, and ν3=857 cm-1. Seven bands in the spectrum were recorded at high resolution and rotationally analyzed, providing excited state molecular constants. The upper state vibrational and rotational bending levels were fitted to a semirigid bender model to obtain the equilibrium geometry and the potential energy barrier to linearity. Due to correlations in the parameters, it was necessary to fix the bond angle at the ab initio value of 114.5°. The resulting fitted model yielded re(Si–F)=1.602 Å, re(Si–H)=1.548 Å with a potential energy barrier to linearity of 9130 cm-1. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Spectra of jet-cooled germanium difluoride were obtained by the reaction of germanium metal and fluorine in the throat of a heated supersonic jet. Two band systems at 331–305 and 231–224 nm were observed by laser-induced emission excitation spectroscopy. On the basis of high quality ab initio predictions of the energies, geometries and vibrational frequencies of the lower electronic states, the two band systems are assigned as a 3B1–X 1A1 and A 1B1–X 1A1, respectively. The T1–S0 spectrum consists of a long, well-resolved progression in the excited state bending frequency with ν2’ = 192.2 cm-1 and T00=30 582.1 cm-1. The S1–S0 spectrum is a complex tangle of vibronic structure accompanied by a rising background. A partial analysis of the vibronic structure gave ν2’ = 159.6 cm-1 and T00=43 860.9 cm-1. The spectra are very similar to the analogous band systems of GeCl2. [ABSTRACT FROM AUTHOR]

An extensive vibronic band system of the FS2 radical in the 700–485 nm region has been observed for the first time by laser-induced fluorescence (LIF) probing of the products of the reaction of F2 with various sulfur-containing compounds. Jet-cooled spectra were obtained by reacting fluorine with COS, H2S, or CS2 in the body of a continuous supersonic jet, just prior to expansion. Vibrational analysis of the jet spectra gives ν1’ = 768, ν2’ = 217, ν3’ = 495, ν1‘ = 705, ν2‘ = 293, ν3‘ = 684, and T00=14 922 cm-1, all in good agreement with ab initio predictions for the FS2 free radical. A rotational analysis of the high-resolution spectrum of the 350 band shows that the transition is polarized out-of-plane, with spin splittings characteristic of a molecule with a single unpaired electron. The rotational constants and spin–rotation interaction constants were determined with good precision for the ground and excited states. By combining the rotational constants with our ab initio estimates of the S–F bond length, the following structural parameters were obtained: r(S–F)=1.651 Å, r(S–S)=1.865(5) Å, and θ(FSS)=109.1(1)° for the X 2A‘ state and r(S–F)=1.642 Å, r(S–S)=2.105(5) Å, and θ(FSS)=97.6(1)° for the A 2A’ state. [ABSTRACT FROM AUTHOR]

A vibronically and partially rotationally resolved electronic spectrum of chlorofluorocarbene has been observed using the technique of pyrolysis jet spectroscopy. A vibrational analysis has established the origin of the electronic transition to be at 25 277.8 cm-1. The pattern of isotope effects for the CF35Cl and CF37Cl species was used to confirm the vibrational assignments. The rotational analysis of three bands established that the transition moment is perpendicular to the molecular plane and can be assigned as A 1A‘–X 1A’. The ground and excited state geometries were derived from the rotational analysis and compared to ab initio predictions. The transition was found to be the direct analog of the 600 nm band system of dichlorocarbene. [ABSTRACT FROM AUTHOR]

Spectra of jet-cooled germanium dichloride were obtained by pyrolysis of trichlorogermane in the throat of a supersonic jet. Laser-induced emission excitation spectra were recorded for the weak 450–400 nm and strong 320–300 nm band systems, both of which were vibrationally analyzed. Ab initio predictions of the excited state geometries, vibrational frequencies, and excitation energies were made to aid in assigning the spectra. The strong ultraviolet band system is assigned as A 1B1–X 1A1 with upper state vibrational frequencies of ν1=354 cm-1 and ν2=104 cm-1. It is the direct analog of the 580–440 nm band system of dichlorocarbene. The weaker band system in the visible is shown to be the a 3B1–X 1A1 transition, with upper state vibrational frequencies of ν1=393 cm-1 and ν2=118 cm-1. This is the first report of direct laser-induced phosphorescence detection of the excited triplet state of any of the carbene or heavier carbene analogs. [ABSTRACT FROM AUTHOR]

The A 1A‘–X 1A’ band systems of HCOCN and DCOCN have been studied in depth using the technique of pyrolysis jet spectroscopy. Ab initio predictions of molecular structures and vibrational frequencies for the ground and excited state were made, to assist in the assignment of the vibronic structure in the laser-induced fluorescence (LIF) spectra. A resolved emission spectrum obtained by laser population of the 81 level of the excited state was analyzed to establish the ground state vibrational frequencies. Both the LIF and resolved emission spectra are consistent with a substantial elongation of the C=O bond on excitation, and smaller changes in the other geometric parameters. The excited state inertial defect, the pattern of bands involving the antisymmetric vibrations, and the ab initio predictions all suggest that the excited state structure is only slightly nonplanar. A rotational analysis of the 910 band of the DCOCN LIF spectrum has provided the first experimental rotational constants for the ground state of formyl cyanide-d1, which should be useful in attempts to observe the microwave spectrum. The uv spectrum of formyl cyanide is found to be remarkably similar to that of propynal. [ABSTRACT FROM AUTHOR]

Jet-cooled, laser induced phosphorescence (LIP) excitation spectra of thioacetaldehyde (CH3CHS, CH3CDS, CD3CHS, and CD3CDS) have been observed in the 15 800–17 300 cm-1 region in a continuous pyrolysis jet. The responsible electronic transition, T1 ← S0, a 3A‘← X 1A’, results from an n→π* electron promotion and gives rise to a pattern of vibronic bands that can be attributed to activity of the methyl torsion and the aldehyde hydrogen out-of-plane wagging modes. Potential and kinetic energy surfaces were mapped out for the aldehyde wagging (α) and the torsional (Θ) internal coordinates by using 6–31G* Hartree–Fock calculations in which the structural parameters were fully relaxed. The potential and kinetic energy data points were fitted to double Fourier expansions in α and Θ and were incorporated into a two-dimensional Hamiltonian operator. The spectrum was simulated from the transition energies and the Franck–Condon factors and was compared to the observed jet cooled LIP spectra. It was concluded that while the RHF procedure gives a good description to the ground state dynamics, the triplet state surface generated by the UHF method is too bumpy and undulating. [ABSTRACT FROM AUTHOR]

Jet-cooled, laser-induced phosphorescence excitation spectra (LIP) of thioacetone (CH3)2CS/(CD3)2CS have been recorded over the region 16 800–18 500 cm-1 using the pyrolysis jet spectroscopic technique. The responsible electronic transition, T1←S0, a 3A‘←X 1A1, results from an n→π* electron promotion and gives rise to a pattern of vibronic bands that were attributed to activity of the methyl torsion and the sulphur out-of-plane wagging modes. The intensities of the torsional and wagging progressions in the excitation spectra were interpreted in terms of a C2v–Cs molecular distortion of the triplet molecule from its singlet ground state equilibrium structure. A complete unrestricted Hartree–Fock (UHF) ab initio molecular orbital (MO) structural optimization of the T1 state predicted that the sulphur was displaced by 27.36° from the molecular plane and the methyl groups were rotated by 10.93° in clockwise–counterclockwise directions. Restricted Hartree–Fock (RHF) calculations were used to generate the V(θ1,θ2) potential surface governing methyl rotation for the S0 state. This was incorporated into a two-dimensional Hamiltonian, symmetrized for the G36 point group and solved variationally for the torsional frequencies. The calculated frequencies of 159.97/118.94 for the ν17(b1) mode of S0 (CH3)2CS/(CD3)2CS were found to agree with the experimental values, 153.2/114.7 cm-1. [ABSTRACT FROM AUTHOR]

Global analysis of a set of room temperature transient absorption spectra of Rhodobacter sphaeroidesreaction centers, recorded in wide temporal and spectral ranges and triggered by femtosecond excitation of accessory bacteriochlorophylls at 800 nm, is presented. The data give a comprehensive review of all spectral dynamics features in the visible and near UV, from 330 to 700 nm, related to the primary events in the Rb. sphaeroidesreaction center: excitation energy transfer from the accessory bacteriochlorophylls (B) to the primary donor (P), primary charge separation between the primary donor and primary acceptor (bacteriopheophytin, H), and electron transfer from the primary to the secondary electron acceptor (ubiquinone). In particular, engagement of the accessory bacteriochlorophyll in primary charge separation is shown as an intermediate electron acceptor, and the initial free energy gap of ∼40 meV, between the states P+BA−and P+HA−is estimated. The size of this gap is shown to be constant for the whole 230 ps lifetime of the P+HA−state. The ultrafast spectral dynamics features recorded in the visible range are presented against a background of results from similar studies performed for the last two decades. [ABSTRACT FROM AUTHOR]