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 2 AЈ←X 2 AЉ band and a higher-energy band system assigned to the C 2 AЉ←X 2 AЉ band were observed. Photofragment mass distributions indicate that N 2 loss is the primary dissociation pathway. Translational energy distributions reveal a resolved vibrational structure of the N 2 fragment, suggesting that the HNCN radical first isomerizes to a cyclic HCN 2 intermediate. A dissociation mechanism is proposed in which electronically excited HNCN undergoes internal conversion to the ground state, followed by isomerization to cyclic HCN 2 and dissociation through a tight three-center transition state. The HNCN bond dissociation energy D 0 and heat of formation ⌬ f H 0 ͑HNCN͒ were determined to be 2.80Ϯ0.03 eV and 3.35Ϯ0.03 eV, respectively.

The photodissociation of mass-selected linear carbon clusters (C n , n ) 4-6) is studied using fast beam photofragment translational spectroscopy. The photofragment yield (PFY) spectra consist of several continua spanning the whole visible and ultraviolet region. The product mass distributions for dissociation of C n clusters are dominated by C 3 and its partner fragment C n-3 , although some minor channels are also identified for dissociation of C 4 and C 5 clusters. Translational energy P(E T ) distributions for the C 3 + C n-3 channel were measured at several photolysis energies. The PFY spectra and P(E T ) distributions indicate that multiphoton dissociation occurs at photon energies below the dissociation threshold and that both single-photon and multiphoton dissociation occur above the threshold. The one-photon components of the P(E T ) distributions can be modeled by phase space theory (PST), suggesting that photoexcitation is followed by internal conversion to the ground state. The PST analysis yields dissociation energies for C n f C 3 + C n-3 in reasonable agreement with recent Knudsen effusion mass spectrometry measurements.

The photodissociation dynamics of bare I

The spectroscopy and dissociation dynamics of I 3 Ϫ were investigated using fast beam photofragment translational spectroscopy. The photofragment yield of I 3Ϫ from 420 to 240 nm was measured, yielding two broadbands at the same energies as in the absorption spectrum of I 3 Ϫ 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( 2 P 3/2 )ϩI 2 Ϫ and I*( 2 P 1/2 )ϩI 2 Ϫ . The 1:1 mass channel is from symmetric three-body dissociation to I Ϫ ϩ2I.

The spectroscopy and photodissociation dynamics of the à 3 ⌸ and B 3 ⌺ Ϫ states of the CNN radical have been investigated by fast beam photofragment translational spectroscopy. Vibronic transitions located more than 1000 cm Ϫ1 above the à 3 ⌸←X 3 ⌺ Ϫ origin were found to predissociate. Photofragment yield spectra for the B 3 ⌺ Ϫ ←X 3 ⌺ Ϫ band between 40 800 and 45 460 cm Ϫ1 display resolved vibrational progressions with peak spacing of Ϸ1000 cm Ϫ1 corresponding to symmetric stretch 1 0 n and combination band 1 0 n 3 0 1 progressions. Ground state products C(3 P)ϩN 2 were found to be the major photodissociation channel for both the à 3 ⌸ and B 3 ⌺ Ϫ states. The translational energy distributions for the à 3 ⌸ state are bimodal with high and low translational energy components. The distributions for the B 3 ⌺ Ϫ state reveal partially resolved vibrational structure for the N 2 photofragment and indicate extensive vibrational and rotational excitation of this fragment. These results suggest that bent geometries are involved in the dissociation mechanism and provide more accurate values: ⌬ f H 0 ͑CNN͒ϭ6.16Ϯ0.05 eV and ⌬ f H 298 ͑CNN͒ϭ6.15Ϯ0.05 eV. These values, coupled with recent D 0 ͑RH͒ and D 298 ͑RH͒ values from Clifford et al. ͓J. Phys. Chem. 102, 7100 ͑1998͔͒, yield ⌬ f H 0 ͑HCNN͒ϭ5.02Ϯ0.18 eV, ⌬ f H 298 ͑HCNN͒ϭ4.98Ϯ0.18 eV, ⌬ f H 0 ͑H 2 CNN͒ϭ3.09Ϯ0.21 eV, and ⌬ f H 0 ͑H 2 CNN͒ϭ3.09Ϯ0.21 eV.

The photodissociation dynamics of I 3 Ϫ from 390 to 290 nm ͑3.18 to 4.28 eV͒ have been investigated using fast beam photofragment translational spectroscopy in which the products are detected and analyzed with coincidence imaging. At photon energies р3.87 eV, two-body dissociation that generates I Ϫ ϩI 2 (A 3 ⌸ 1u ) and vibrationally excited I 2 Ϫ (X 2 ⌺ u ϩ )ϩI( 2 P 3/2 ) is observed, while at energies у3.87 eV, I*( 2 P 1/2 )ϩI 2 Ϫ (X 2 ⌺ u ϩ ) is the primary two-body dissociation channel. In addition, three-body dissociation yielding I Ϫ ϩ2I( 2 P 3/2 ) photofragments is seen throughout the energy range probed; this is the dominant channel at all but the lowest photon energy. Analysis of the three-body dissociation events indicates that this channel results primarily from a synchronous concerted decay mechanism.

This paper describes the design and experimental application of an optical system to perform schlieren measurements in the curved geometry of the cylinder of an optically accessible internal combustion engine. Key features of the system are a pair of cylindrical positive meniscus lenses, which keep the beam collimated while passing through the unmodified, thick-walled optical cylinder, and a pulsed, high-power light-emitting diode with narrow spectral width. In combination with a high-speed CMOS camera, the system is used to visualize the fuel jet after injection of hydrogen fuel directly into the cylinder from a high-pressure injector. Residual aberrations, which limit the system's sensitivity, are characterized experimentally and are compared to the predictions of ray-tracing software.

The electronic spectroscopy and photodissociation dynamics of the I 3 radical are investigated with two experimental methods. The ground and several low-lying excited states of the I 3 radical are characterized by photoelectron spectroscopy of I 3Ϫ at 213 nm. Assignments of these states are discussed with reference to recent calculations. In addition, photodissociation of the I 3 radical was investigated at selected photon energies ͑4.59, 4.96, and 5.17 eV͒ by fast radical beam photofragment translational spectroscopy. Two product channels were observed with mass ratios of 1:2 and 1:1, and translational energy ( P(E T )) distributions were measured. The P(E T ) distributions for products with mass ratio 1:2 show that this channel corresponds to I 2 in various electronic states along with atomic I in its 2 P 3/2 or 2 P 1/2 state. The 1:1 channel corresponds primarily to concerted three-body dissociation to three I atoms.