R. H. Judge

Asyrotwin: A 32-bit Windows version of Asyrot, A program for the analysis of high resolution singlet-singlet band spectra of asymmetric tops

Abstract A new version of the Asyrot program has been written to extend the computational limits to J=999 and K a =450 and to include the octic, M K , and N K centrifugal distortion constants. The number of transitions calculated and fitted is limited only by available computer memory. Two new phases have been added to the program so that the parameters of a single state can be refined by forming combination differences and then fitting them, along with any available microwave lines, to the appropriate Hamiltonian. A new Windows interface has been integrated with the Fortran code to greatly facilitate the editing of the input data. A limited Windows based band contour plotting and comparison feature has been added. Two utility features are also available. The first allows for the easy transfer of observed line frequencies to the corresponding transition quantum labels listed in the input file. The second allows for the summation of several calculated spectra, of different bands or isotopomers, into an overall contour which can be compared to an observed spectrum.

The T1(nπ*)←S0 laser induced phosphorescence excitation spectrum of acetaldehyde in a supersonic free jet: Torsion and wagging potentials in the lowest triplet state

The laser induced T1(nπ*)←S0 phosphorescence excitation spectrum of jet‐cooled acetaldehyde has been observed for the first time with a rotating slit nozzle excitation system. The vibronic origins were fitted to a set of levels that were obtained from a Hamiltonian that employed flexible torsion‐wagging large amplitude coordinates. The potential surface extracted from the fitting procedure yielded barriers to torsion and inversion of 609.68 and 869.02 cm−1, respectively. Minima in the potential hypersurface at θ=61.7° and α=42.2° defined the equilibrium positions for the torsion and wagging coordinates. A comparison to the corresponding S1‐state parameters showed that the torsion barrier (in cm−1) does not greatly change, S1/T1=710.8/609.7, whereas the barrier height for the wagging‐inversion barrier increases dramatically, 574.4/869.0.

Torsion–rotation interactions in a two‐top molecule: High resolution S1←S0 electronic spectrum of 2,3‐dimethylnaphthalene

A rotationally resolved fluorescence excitation spectrum of the O00 band in the S1←S0 transition of 2,3‐dimethylnaphthalene has been obtained using a cw laser/molecular beam spectrometer. More than 3000 lines were observed, each exhibiting a width of about 3 MHz. Despite the proximity of the two methyl groups, the observed rotational structure can be interpreted satisfactorily using an uncoupled rotor model. However, extensive torsion–rotation interactions are observed. Accurate measurements of these perturbations are used to determine the effective threefold hindering potentials in both electronic states, V3(S0)=652 cm−1 and V3(S1)=391 cm−1, respectively. Rotor–rotor couplings do influence these barriers as well as the patterns of lines observed in the rotationally resolved spectra of higher vibronic bands.

Computerized simulation and fitting of singlet–triplet spectra of orthorhombic asymmetric tops: Theory and extensions to molecules with large multiplet splittings

Motivated by our recent finding that the singlet–triplet bands of selenoformaldehyde involve an upper state with large zero field splittings, we have extended the theory and written a program for predicting and fitting such rotationally resolved spectra. Triplet state matrix elements for a case (A) basis have been developed, including corrections for centrifugal and spin–centrifugal distortion. The full Hamiltonian matrix has been symmetry adapted, simplifying the problem to four individual matrices of approximately equal size for molecules of orthorhombic symmetry. Diagonalization of these matrices yields triplet state energies that are in agreement with previous treatments using a basis in which the spin splittings are small relative to the rotational intervals. Methods have been developed for sorting the eigenvalues and assigning quantum labels regardless of the magnitude of the spin splittings. The calculation of the relative intensities of the rotational lines within a band has been programmed using ...

Spectroscopy and predissociation of the 3A2 electronic state of ozone 16O3 and 18O3 by high resolution Fourier transform spectrometry

A high resolution Fourier transform spectrometry analysis of the rotational structure of the 2(0)1 absorption bands of the 3A2<--X1A1 Wulf transition for the isotopomers 16O3 and 18O3 of the ozone molecule is presented. These bands are very intense compared to the 0(0)0 bands but the predissociation is so strong that the main sub-bands appear as continuous contours. Isolated lines and band contour methods are used together to analyse these two rovibrational bands. The lines corresponding to the F2 component are generally the most intense and isolated. Our data sets for the (0 1 0) level of the 3A2 state are limited to about 102 weakly or unperturbed rotational lines for the 2(0)1 of 16O3 in the range 9620-10,140 cm(-1) and 123 weakly or unperturbed rotational lines for the same band of 18O3. Using for each of them the well-defined ground state parameters, we obtained a standard deviation of about 0.035 cm(-1) in the fit to the lines for 16O3 and 0.027 cm(-1) in the case of 18O3. The rotational constants A, B and C, the three rotational distortion terms deltaK, deltaJK and deltaJ, the spin-rotation constants a0, a and b have been successfully calculated for 16O3 and 18O3 while the spin-spin constants were fixed to their respective values obtained for the origin bands. As is the case for the 0(0)0 band, we have a partial agreement with the isotopic laws for the rotational constants. The geometrical parameters of the (0 1 0) level of 3A2 state for the two isotopomers are close, r = 1.357 A, theta = 100.7 degrees for 18O3 and r = 1.352 A and theta = 100.0 degrees for 16O3. The origin of the 2(0)1 band of 18O3 is red shifted by 7.06(4) cm(-1) with respect to 16O3 2(0)1 band and the two bending mode quanta are, respectively, 528.99(9) and 501.34(7) cm(-1). A preliminary qualitative analysis of the predissociation is given in the particular case of the F2 spin component of 16O3 for 0(0)0 and 2(0)1 bands by the measurement of shifts of positions of some rovibrational levels and the evolution of predissociation broadenings in (Q)Q2 branches. We justify the existence of perturbations in the rovibrational levels of 3A2 state through different interaction types: with the dissociation continuum of the same electronic state or with high vibrational repulsive or weakly bound levels of the ground state.

The hybrid rotational components in the 000 origin band of the ÃA‘(S1)←X̃A’(S0) transition in acetaldehyde

The 000 origin band of the S1←S0, electronic transition that results from n→π* electron promotion has been observed under molecular beam conditions with a pulse amplified ring laser. At low temperatures, ∼0.7 K, the spectrum consisted of 11 lines that originated from Ka‘=0 and J‘=0 or 1 rotational levels. A rotational analysis revealed that the transition between the a1–a1 torsional levels gives rise to a c‐type band, whereas the e–e levels are connected by a hybrid transition that has components along the a, b, and c principal axes. The fluorescence emission from the e levels was greatly reduced at temperatures above 3 K. The interpretation of this photophysical effect requires an intermolecular collision within the molecular beam that quenches the fluorescence from the S1 state.

Single vibronic level emission spectroscopy of jet-cooled HSiF and DSiF

Using the technique of single vibronic level emission spectroscopy, the ground state vibrational manifolds of jet-cooled HSiF and DSiF have been studied. The radicals were produced in a pulsed electric discharge jet using trifluorosilane (HSiF3 or DSiF3) as the precursor. The gas phase ground state harmonic vibrational frequencies of both isotopomers have been determined for the first time. A normal coordinate analysis using the vibrational frequencies and literature values for the centrifugal distortion constants allowed the determination of all six ground state force constants. Our previous ground state rotational constants have been combined with the calculated harmonic contributions to the α constants to obtain an average (rz) structure and an estimate of the equilibrium (rez) structure. The reliability of the force constants has been evaluated by Franck–Condon simulations of the emission spectra and comparisons of the calculated and experimentally determined inertial defects.

Computer-assisted analysis of singlet-triplet rotational spectra: application to case (A), case (B) and case (AB) coupling cases in polyatomic molecules

A non-graphical, character-based computer program for the least-squares analysis of the high-resolution rotational spectra of singlet-triplet transitions in orthorhombic molecules has been developed under the ANSI C programming language. The program is applicable to molecules which show either Hunds case (A) or case (B) or case (AB) type splittings. Up to 22 rotational, spin, centrifugal distortion and spin-centrifugal distortion constants may be varied. Only constants of the excited state may be fitted. The program has been tested under the HP-UX and DOS/Windows environments. J-values up to 255 can be accommodated in the UNIX version of the program while the DOS/Windows version is limited to J = 50. A compiled and linked executable file is available for the DOS/Windows version for those without access to a C compiler. A limited band contour capability is provided through the production of an ASCII file of frequency versus intensity for the calculated spectrum.

Selenoformaldehyde: Rotational analysis of the Ã1A2-X̃1A1 735 nm band system of H2C78Se, H2C80Se, and D2C80Se from high-resolution laser fluorescence excitation spectra

High-resolution laser fluorescence excitation spectra of vibronic bands in the A1A2-X1A1 system of H2C78Se, H2C80Se, and D2C80Se have been observed with Doppler-limited resolution. Three bands have been analyzed, yielding upper state molecular constants and improved ground state constants. The electronic transition is shown to be singlet-singlet in nature and the band polarizations are consistent with previous vibronic assignments. Erratic perturbations are observed in all three bands. The derived excited state r0 structure is similar to that of H2CS, suggesting that selenoformaldehyde adopts a near-planar equilibrium structure in the excited state.

Contribution to the analysis of the predissociated rovibronic structure of the symmetric isotopomers and of ozone near 10,400 cm−1: and

The absorption spectrum of ozone was recorded at low temperatures (down to -135 degrees C) by high resolution Fourier transform spectrometry and intra cavity laser absorption spectroscopy (ICLAS) near 10,400 cm-1. A preliminary analysis of the rotational structure of the absorption spectra of 16O3 and 18O3 shows that this spectral region corresponds to a superposition of two different electronic transitions, one with a very broad rotational structure, showing for the first time the asymmetric stretching frequency mode nu3 of the electronic state 3A2, the other formed by a completely diffuse band, probably the 2(1)(0) band of a new transition due to the triplet electronic state 3B2. Predissociation effects induce large broadening of the rotational lines for the transition centered at 10,473 cm-1 identified as the 3(2)(0) band of the 3A2 <-- X1A1 electronic transition. The rotational structure cannot be analyzed directly but instead the band contour method was used to confirm the symmetry of the transition and to estimate the spectroscopic constants for the 16O isotopomer. The origin of the band is at 10,473 +/- 3 cm-1 and the value of the 16O3(3A2) antisymmetric stretching frequency mode is equal to 460 +/- 2 cm-1. We believe that the diffuse band is due to the 3B2 state and is located at about 10,363 +/- 3 cm-1 for 16O3 and 10,354 +/- 3 cm-1 for 18O3. The isotopic rules confirm the different results obtained for 18O3 and 16O3.