J. Karolczak

Pyrolysis jet spectroscopy

Abstract Pyrolysis jet spectroscopy, in which precursor species are heated to pyrolysis temperatures just prior to expansion from the nozzle of a supersonic free jet, is demonstrated to be a viable technique for the production and characterization of rotationally cold transient molecules and free radicals.

The structure, spectroscopy, and excited state predissociation dynamics of GeH2

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) A, θ″=91.2(8)° and r′=1.553(12) A, 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 transit...

Direct laser-induced emission detection of the S1 and T1 states of germanium dichloride : pyrolysis jet spectroscopy and ab initio studies

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.

Pyrolysis jet spectroscopy of dichlorosilylene

Abstract The laser-induced fluorescence spectrum of SiCl2 in the near-ultraviolet has been recorded using the pyrolysis jet technique. Pyrolysis of SiHCl3 has been shown to be an effective method of producing SiCl2 for spectroscopic studies. The spectra show resolved rotational subband structure and chlorine isotope effects. The fundamental vibrational frequencies are ν″1 = 521.6 cm−1, ν″2 = 200.6 cm−1, ν′1 = 428.9 cm−1 and ν′2 = 149.8 cm−1, and the band origin is at 30013.5 cm−1. By comparing with the analogous spectra of CCl2 and GeCl2, the band system is assigned to the A 1B1- X 1A1 electronic transition.

Chemical reaction jet spectroscopy, molecular structure, and the bending potential of the à 1A″ state of monofluorosilylene (HSiF)

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 A, re(Si–H)=1.548 A with a potential energy barrier to linearity of 9130 cm−1.

The electronic spectrum of chlorofluorocarbene

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.

Pyrolysis jet spectroscopy and ab initio studies of the S1 and T1 states of germanium difluoride

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.

Pyrolysis jet spectroscopy: The S1–S0 band system of formyl cyanide, HCOCN, and DCOCN

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...

High-resolution absorption and pyrolysis jet spectroscopy of the 000 band of the Ã1A″ ← X̃1A′ (n − π∗) electronic transition of formyl cyanide, HCOCN

Abstract High-resolution spectrographic absorption and pyrolysis jet spectra of the 000 band of the A 1 A″ ← X 1 A′ system of formyl cyanide have been recorded. The analysis of the spectra clearly establishes that the band is a type C perpendicular transition. Upper state molecular constants were obtained by a simultaneous fitting of the jet and absorption data. The origin of the electronic transition is at 26 283.396 cm−1. The upper state is almost completely free of detectable perturbations.

Spectroscopic detection and characterization of the FS2 free radical

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 len...