Ryan T. Bise

Fast beam photodissociation spectroscopy and dynamics of the vinoxy radical

The spectroscopy and photodissociation dynamics of the vinoxy (CH2CHO) radical B(2A″) ←X(2A″) transition have been investigated by fast beam photofragment translational spectroscopy. We show conclusively that excitation to the B state is followed by predissociation, even for the origin transition. Two photodissociation channels are observed: (1) CH3+CO, and (2) H+CH2CO, with a branching ratio of ≈1:4. The form of the translational energy distributions imply a significant exit barrier to formation of CH3+CO, and a considerably smaller barrier for H+CH2CO formation. Dissociation ultimately proceeds by internal conversion to the ground electronic state; the internal conversion rate appears to be significantly enhanced by a curve crossing with either the A(2A) or C(2A) states. Ab initio calculations of critical points on the global potential energy surfaces aid in determining the dissociation mechanism. We present a simple model for dissociation over a barrier, the statistical adiabatic impulsive model, w...

Photodissociation spectroscopy and dynamics of the HCCO free radical

The photodissociation spectroscopy and dynamics of the HCCO radical have been investigated using fast radical beam photofragment translational spectroscopy. An electronic band with origin at 33 424 cm−1 has been identified. This band exhibits rotational resolution near the band origin, but the well-defined rovibronic structure is homogeneously broadened at higher photon energies. Based on the rotational structure this band is assigned to the B 2Π←X 2A′′ transition. Photofragment translational energy and angular distributions were obtained at several excitation energies. At excitation energies close to the origin, the excited, spin-forbidden CH(a 4Σ−)+CO channel dominates, while the ground state CH(X 2Π)+CO channel is the major channel at higher photon energies. The translational energy distributions provide evidence of competition between intersystem crossing and internal conversion dissociation mechanisms, with some evidence for nonstatistical dynamics in the CH(X 2Π)+CO channel. This work yields an im...

Ultraviolet photodissociation of the HCCO radical studied by fast radical beam photofragment translational spectroscopy

The ultraviolet photolysis of jet‐cooled mass‐selected ketenyl radicals has been investigated using the technique of fast radical beam photofragment translational spectroscopy. The C2Π(2A″)–X2A″ photofragment yield cross section spans 33 400–48 000 cm−1 and exhibits resolved resonances and broad continua. Dissociation produces both ground and excited state CH radicals in association with ground state CO fragments; there is no evidence for H atom elimination. Analysis of the photofragment kinetic energy release spectra yield a value for the C–C bond dissociation energy and heat of formation of HCCO: D0(HC–CO)=3.14±0.03 eV (72.4±0.7 kcal/mol) and ΔHf,00(HCCO)=1.82±0.03 eV (42.0±0.7 kcal/mol).

PHOTODISSOCIATION SPECTROSCOPY AND DYNAMICS OF THE METHYLTHIO RADICAL (CH3S)

The photodissociation spectroscopy and dynamics of the CH3S and CD3S radicals have been investigated using fast radical beam photofragment spectroscopy of the A 2A1←X 2E electronic band (T0≅26 400 cm−1) and an unstructured band near 45 600 cm−1. At all energies, only one major channel, CH3(X 2A2″)+S(3Pj), was observed. Photofragment yield spectra for the A 2A1←X 2E electronic band show resolved vibrational progressions extending well beyond those seen in laser-induced fluorescence studies of this band. Photofragment translational energy distributions yield the S(3Pj) fine-structure distribution for each vibrational level of the CH3 product. Photofragment angular distributions were found to be highly anisotropic (β=−0.2 to −1.0±0.1) with increasing anisotropy at higher photon energies. The results yield a refined heat of formation for CH3S (1.346±0.018 eV) as well as the mechanism by which the A 2A1 state is predissociated. Results at 45 600 cm−1 imply that dissociation occurs on the repulsive B 2A2 state.

Laser-induced fluorescence spectroscopy of the ketenyl radical

We report the first laser-induced fluorescence (LIF) excitation spectrum of the ketenyl radical, HCCO, which is produced by the 193 nm photolysis of ketene in a free jet expansion. A series of vibronic bands in the B 2Π–X 2A′′ system are observed. The LIF band positions and rotational structures are in excellent agreement with those of a recent photofragment yield (PFY) spectrum [D. L. Osborn et al., J. Chem. Phys. (to be published)] from the origin at 33 424 cm−1 to 35 100 cm−1 . At higher energies the LIF spectrum breaks off sharply due to a rapid increase in the rate of predissociation. The lifetime of the vibrationless level of the B state is less than 1 ns; the estimated quantum yield of fluorescence is ∼10−3.

Photodissociation of triplet and singlet states of the CCO radical

The triplet and singlet states of the ketenylidene (CCO) radical are investigated using fast radical beam photofragment translational spectroscopy, in which CCO is generated by laser photodetachment of CCO− and subsequently photodissociated, and anion photoelectron spectroscopy. In the photodissociation experiment, two bands in which the upper state of CCO predissociates are studied. Photodissociation from excitation of the A 3Π–X 3Σ− band in CCO is observed from 16 666–23 529 cm−1; resonances are observed and assigned to excited vibrational levels involving all three vibrational modes. We also report the first observation of the c 1Π–a 1Δ band in CCO. Here, the a 1Δ state of CCO is generated by laser photodetachment at higher photon energy than was used to generate the X 3Σ state. The c 1Π state is approximately located by photoelectron spectroscopy of CCO−, and the photodissociation experiment shows that the origin of the c 1Π–a 1Δ band occurs around 17 170 cm−1. Kinetic-energy release spectra from...

Photodissociation dynamics of the singlet and triplet states of the NCN radical

The spectroscopy and photodissociation dynamics of the NCN radical have been investigated by fast beam photofragment translational spectroscopy. The B 3Σu−←X 3Σg−, c 1Πu←a 1Δg, and d 1Δu←a 1Δg transitions were examined. The major dissociation products for the B 3Σu− and c 1Πu states are N2(X 1Σg+)+C(3P), while the d 1Δu state dissociates to N2(X 1Σg+)+C(1D). The dissociation channel, N(4S)+CN(X 2Σ+) is observed for the B 3Σu− state at photon energies greater than 4.9 eV, where it comprises ≈25±10% of the total signal. At all photon energies, the photofragment translational energy distributions show a resolved progression corresponding to the vibrational excitation of the N2 photofragment. The rotational distributions of the molecular fragments suggest that the dissociation pathway for the N2 loss channel involves a bent transition state while the N+CN photofragments are produced via a linear dissociation mechanism. The P(ET) distributions provide bond dissociation energies of 2.54±0.030 and 4.5...

Photodissociation and photoisomerization pathways of the HNCN free radical

The photodissociation spectroscopy and dynamics of the HNCN free radical have been investigated by fast beam photofragment translational spectroscopy. Predissociative transitions for both the B 2A′←X 2A″ band and a higher-energy band system assigned to the C 2A″←X 2A″ band were observed. Photofragment mass distributions indicate that N2 loss is the primary dissociation pathway. Translational energy distributions reveal a resolved vibrational structure of the N2 fragment, suggesting that the HNCN radical first isomerizes to a cyclic HCN2 intermediate. A dissociation mechanism is proposed in which electronically excited HNCN undergoes internal conversion to the ground state, followed by isomerization to cyclic HCN2 and dissociation through a tight three-center transition state. The HNCN bond dissociation energy D0 and heat of formation ΔfH0(HNCN) were determined to be 2.80±0.03 eV and 3.35±0.03 eV, respectively.

Photodissociation dynamics of the triiodide anion (I3

The spectroscopy and dissociation dynamics of I3− were investigated using fast beam photofragment translational spectroscopy. The photofragment yield of I3− from 420 to 240 nm was measured, yielding two broadbands at the same energies as in the absorption spectrum of I3− in solution. Photodissociation dynamics measurements performed with two-particle time-and-position sensitive detection revealed two product mass channels having photofragment mass ratios of 1:2 and 1:1. Both channels were seen at all photolysis wavelengths. Translational energy distributions show that the 1:2 products are from a combination of I(2P3/2)+I2− and I*(2P1/2)+I2−. The 1:1 mass channel is from symmetric three-body dissociation to I−+2I.

Photodissociation dynamics of the CNN free radical

The spectroscopy and photodissociation dynamics of the A 3Π and B 3Σ− states of the CNN radical have been investigated by fast beam photofragment translational spectroscopy. Vibronic transitions located more than 1000 cm−1 above the A 3Π←X 3Σ− origin were found to predissociate. Photofragment yield spectra for the B 3Σ−←X 3Σ− band between 40 800 and 45 460 cm−1 display resolved vibrational progressions with peak spacing of ≈1000 cm−1 corresponding to symmetric stretch 10n and combination band 10n301 progressions. Ground state products C(3P)+N2 were found to be the major photodissociation channel for both the A 3Π and B 3Σ− states. The translational energy distributions for the A 3Π state are bimodal with high and low translational energy components. The distributions for the B 3Σ− state reveal partially resolved vibrational structure for the N2 photofragment and indicate extensive vibrational and rotational excitation of this fragment. These results suggest that bent geometries are involved in the d...