Pulsed discharge jet electronic spectroscopy of the aluminum dicarbide (AlC2) free radical.

Laser-induced fluorescence and wavelength resolved emission spectra of the C ̃(2)B(2)-X̃ (2)A(1) band system of the gas phase aluminum dicarbide free radical have been obtained using the pulsed discharge jet technique. The radical was produced by electron bombardment of a precursor mixture of trimethylaluminum in high-pressure argon. The three vibrational frequencies of T-shaped AlC(2) have been determined in both the combining states along with several of the anharmonicity constants. The 0(0)(0) band has been recorded with high resolution and rotationally analyzed. The spectrum is complicated by partially resolved spin-rotation and aluminum hyperfine splittings. Where necessary, we have fixed the spin-rotation constants used in the rotational analysis at the values predicted by density functional theory. The derived molecular structures are: r(0)('')(C-C) = 1.271(2) Å, r(0)('')(Al-C) = 1.926(1) Å, θ(")(C-Al-C) = 38.5(2)°, r(0)(')(C-C) = 1.323(2) Å, r(0)(')(Al-C) = 1.934(1) Å, and θ(')(C-Al-C) = 40.0(2)°. Unlike SiC(2), aluminum dicarbide shows no spectroscopic evidence of facile isomerization to the linear structure in the ground electronic state.

High resolution study of spin-orbit mixing and the singlet-triplet gap in chlorocarbene: stimulated emission pumping spectroscopy of CH(35)Cl and CD(35)Cl.

We report on high resolution studies of spin-orbit mixing and the singlet-triplet gap in a prototypical halocarbene, CHCl, using stimulated emission pumping (SEP) spectroscopy from the A (1)A(") state. Results are reported for two isotopomers, CH(35)Cl and CD(35)Cl. We have obtained rotationally resolved spectra for the majority of X (1)A(') levels lying between 0 and 6000 cm(-1) above the zero-point level that were previously observed under low resolution in single vibronic level emission studies and several new levels that were previously unobserved or unresolved. In addition, SEP spectra were obtained for six a (3)A(") levels in CH(35)Cl and three levels in CD(35)Cl. The derived term energies and rovibrational parameters of the X (1)A(') and a (3)A(") states are in good agreement with theory. The a (3)A(") triplet spin-spin parameter is vibrational state dependent, and dominated by a second-order contribution from spin-orbit coupling with nearby X (1)A(') levels; it therefore provides a sensitive probe of spin-orbit mixing in this system. An analysis of three pairs of interactions between specific a (3)A(") and X (1)A(') levels in CH(35)Cl affords a pure electronic spin-orbit coupling element of 150 cm(-1), in good agreement with theoretical expectations. The derived singlet-triplet gaps, which are the most precise determined to date for any carbene, are compared with the predictions of high level ab initio theory.

High resolution probe of spin-orbit coupling and the singlet-triplet gap in chlorocarbene.

Among the most important of chemical intermediates are the carbenes, characterized by a divalent carbon that generates low-lying biradical (triplet) and spin-paired (singlet) configurations with unique chemical reactivities. The "holy grail" of carbene chemistry has been determining the singlet-triplet gap and intersystem crossing rates. We report here the first high resolution spectra of singlet-triplet transitions in a prototypical singlet carbene, CHCl, which probe in detail the triplet state structure and spin-orbit coupling with the ground singlet state. Our spectra reveal a pronounced vibrational state dependence of the triplet state spin-spin splitting parameter, which we show is a sensitive probe of spin-orbit coupling with nearby singlet states. The parameters derived from our spectra, including a precise determination of the singlet-triplet energy gap, are in excellent agreement with recent ab initio calculations.

Jet-cooled phosphorescence excitation spectrum of the T1(n,pi*) <-- S0 transition of 2-cyclopenten-1-one.

The T1(n,pi*) <-- S0 transition of 2-cyclopenten-1-one (2CP) was investigated by using phosphorescence excitation (PE) spectroscopy in a free-jet expansion. The origin band, near 385 nm, is the most intense feature in the T1(n,pi*) <-- S0 PE spectrum. A short progression in the ring-bending mode (nu'(30)) is also observed. The effective vibrational temperature in the jet is estimated at 50 K. The spectral simplification arising from jet cooling helps confirm assignments made previously in the room-temperature cavity ringdown (CRD) absorption spectrum, which is congested by vibrational hot bands. In addition to the origin and nu'(30) assignments, the jet-cooled PE spectrum also confirms the 28(0)(1) (C=O out-of-plane wag), 29(0)(1) (C=C twist), and 19(0)(1) (C=O in-plane wag) band assignments that were made in the T1(n,pi*) <-- S0 room-temperature CRD spectrum. The temporal decay of the T1 state of 2CP was investigated as a function of vibronic excitation. Phosphorescence from the v' = 0 level persists the entire time the molecules traverse the emission detection zone. Thus the phosphorescence lifetime of the v' = 0 level is significantly longer than the 2 micros transit time through the viewing zone. Higher vibrational levels in the T1 state have shorter phosphorescence lifetimes, on the order of 2 micros or less. The concomitant reduction in emission quantum yield causes the higher vibronic bands (above 200 cm(-1)) in the PE spectrum to be weak. It is proposed that intersystem crossing to highly vibrationally excited levels of the ground state is responsible for the faster decay and diminished quantum yield. The jet cooling affords partial rotational resolution in the T1(n,pi*) <-- S0 spectrum of 2CP. The rotational structure of the origin band was simulated by using inertial constants available from a previously reported density functional (DFT) calculation of the T1(n,pi*) state, along with spin constants obtained via a fitting procedure. Intensity parameters were also systematically varied. The optimized intensity factors support a model that identifies the S2(pi,pi*) <-- S0 transition in 2CP as the sole source of oscillator strength for the T1(n,pi*) <-- S0 transition.

Single vibronic level emission spectroscopic studies of the ground state energy levels and molecular structures of jet-cooled HGeBr, DGeBr, HGeI, and DGeI.

Single vibronic level dispersed fluorescence spectra of jet-cooled HGeBr, DGeBr, HGeI, and DGeI have been obtained by laser excitation of selected bands of the A (1)A(")-X (1)A(') electronic transition. The measured ground state vibrational intervals were assigned and fitted to anharmonicity expressions, which allowed the harmonic frequencies to be determined for both isotopomers. In some cases, lack of a suitable range of emission data necessitated that some of the anharmonicity constants and vibrational frequencies be estimated from those of HGeClDGeCl and the corresponding silylenes (HSiX). Harmonic force fields were obtained for both molecules, although only four of the six force constants could be determined. The ground state effective rotational constants and force field data were combined to calculate average (r(z)) and approximate equilibrium (r(e) (z)) structures. For HGeBr r(e) (z)(GeH)=1.593(9) A, r(e) (z)(GeBr)=2.325(21) A, and the bond angle was fixed at our CCSD(T)/aug-cc-pVTZ ab initio value of 93.6 degrees . For HGeI we obtained r(e) (z)(GeH)=1.589(1) A, r(e) (z)(GeI)=2.525(5) A, and bond angle=93.2 degrees . Franck-Condon simulations of the emission spectra using ab initio Cartesian displacement coordinates reproduce the observed intensity distributions satisfactorily. The trends in structural parameters in the halogermylenes and halosilylenes can be readily understood based on the electronegativity of the halogen substituent.

An optical-optical double resonance probe of the lowest triplet state of jet-cooled thiophosgene: rovibronic structures and electronic relaxation.

The vibrational structure, rotational structure, and electronic relaxation of the "dark" T1 3A2(n,pi*) state of jet-cooled thiophosgene have been investigated by two-color S2<--T1<--S0 optical-optical double resonance (OODR) spectroscopy, which monitors the S2-->S0 fluorescence generated by S2<--T1 excitation. This method is capable of isolating the T1 vibrational structure into a1, b1, and b2 symmetry blocks. The fluorescence-detected vibrational structure of the Tz spin state of T1 shows that the CS stretching frequency as well as the barrier height for pyramidal deformation are significantly greater in the 3A2(n,pi*) state than in the corresponding 1A2(n,pi*) state. The differing vibrational parameters of the T1 thiophosgene relative to the S1 thiophosgene can be attributed to the motions of unpaired electrons that are better correlated when they are in the excited singlet state than when they are in the triplet state of same electron configuration. A set of T1 structural parameters and the information concerning the T1 spin states have been obtained from least-square fittings of the rotationally resolved T1<--S0 excitation spectrum. The nearly degenerate mid R:x and mid R:y spin states are well removed from mid R:z spin component, indicating that T1 thiophosgene is a good example of case (ab) coupling. The decay of the mid R:z spin state of T1 thiophosgene, obtained from time-resolved S2<--T1<--S0 OODR experiment, is characteristic of strong-coupling intermediate-case decay in which an initial rapid decay is followed by recurrences and/or a long-lived quasiexponential decay.