Brent Koplitz

Iodoethane photolysis: Which C–H bond leads to H‐atom formation?

Results are reported on the 193 and 248 nm photolysis of iodoethane, specifically with respect to H‐atom production. Experiments using selectively deuterated iodoethanes, ICD2CH3 and ICH2CD3, reveal that at 193 nm the carbon–hydrogen bond cleavage is not carbon‐atom specific. However, following photolysis at 248 nm, it is clear that C–H (or C–D) bond dissociation occurs preferentially at the β carbon atom.

Site‐specific branching ratios for H‐atom production from primary haloalkanes photolyzed at 193, 222, and 248 nm

Selectively deuterated compounds are used to investigate the ‘‘site‐specific’’ nature of H‐atom production resulting from the photolysis of primary haloalkanes. The molecules investigated are 1‐iodopropane, 1‐bromopropane, iodoethane, bromoethane, and chloroethane, with photolysis being initiated at 193, 222, and 248 nm. Hydrogen and deuterium isotopes are systematically used to label chemically distinct carbon atoms within a given molecule. H‐ and D‐atom Doppler profiles are generated via two‐photon (121.6+364.7 nm) ionization resonant with Lyman‐α, and the relative H/D ratios are used to quantify the probability for hydrogen production from each carbon site. In general, photolysis of an intermediate, presumably the alkyl radical, is implicated as being a key step in the overall process. When using 248 nm radiation, the photolysis process is dominated by C–H (or C–D) bond cleavage at the β carbon position regardless of the system investigated. In contrast, results using 193 nm excitation display no obvio...

Conclusive evidence for site‐specific C–H bond cleavage resulting from 248 nm photolysis of the ethyl radical

Experiments involving two photolysis lasers and one probe laser demonstrate that 248 nm excimer laser radiation will induce C–H bond cleavage preferentially at the β position in the ethyl radical. To facilitate carbon site labeling, selectively deuterated chloroethanes (ClCH2CD3 and ClCD2CH3) are used as precursor compounds. Two‐photon ionization via resonance with the Lyman‐α transition is used to detect H (or D) atoms. An initial 193 nm photolysis pulse serves to cleave the C–Cl bond in ClCH2CH3, while a second pulse at 248 nm dramatically enhances H‐atom production. Experiments on ClCH2CD3 and ClCD2CH3 clearly show that this enhancement occurs preferentially through carbon–hydrogen bond cleavage at the β carbon site. It is apparent that 248 nm photon absorption by the ethyl radical is an important step in the overall mechanism.

Energetics of pulsed laser ablation species as determined by quadrupole and time-of-flight mass spectrometry

Time-of-flight data from an electron impact quadrupole mass spectrometer (QMS) are used to determine the composition and energy of a neutral plume created by pulsed laser ablation of ZnTe. Velocities of the ablated species are extracted by taking spectra at two distances and measuring the change in arrival time. Results from the QMS are compared to those obtained by 193-nm laser ionization time-of-flight mass spectrometry.

The 193 nm fragmentation and ionization of trimethylaluminum: Evidence for photoinduced α‐hydrogen elimination

Results are reported on the 193 nm excitation of trimethylaluminum under collisionless conditions. Time‐of‐flight mass spectra are monitored at several different excimer laser powers. At relatively low powers, the mass spectra consist solely of masses 27 and 57, presumably the Al+ and Al(CH3)+ 2 ions. At higher laser powers, however, mass 58 is readily observed, suggesting the presence of a reaction channel involving α‐hydrogen elimination to form the AlH(CH3)2 photofragment. This observation is interpreted in the context of recent theoretical calculations by Higashi and Steigerwald [Appl. Phys. Lett. 5 4, 81 (1989)].

Laser-initiated reactivity in constrained gas expansions using trimethylgallium and ammonia as precursors

Abstract The present work combines pulsed lasers and nozzles in order to form GaN-containing species by using metalorganic compounds as precursors. The apparatus consists of a high-vacuum chamber equipped with a specialized dual-source pulsed-nozzle assembly and a quadrupole mass spectrometer (QMS). Ammonia and trimethylgallium are introduced into the high-vacuum chamber via the nozzle assembly. The 193 nm output from an ArF excimer laser is focused into the mixing and reaction region of the nozzle assembly. Reaction products are subsequently mass-analyzed with the QMS and show efficient production of simple GaN-containing species due to laser-assisted growth.

PHOTOLYSIS OF IODOETHANE : ATOMIC HYDROGEN GENERATION

Abstract Results are reported on the photolysis and photoionization of iodoethane under collisionless conditions. Although the halogen—carbon bond is the weakest bond, H-atom production is observed following the photolysis of iodoethane at 193 and 248 nm. The H atoms are probed using two-photon (121.6 + 364.7 nm) ionization, and H-atom Doppler profiles at Lyman-α are presented. Time-of-flight mass spectra and power dependence studies are also reported. Mechanistically, the ethyl radical is implicated as being a key intermediate, and the overall dissociation/ionization behavior is discussed in terms of the different electronic transitions involved with the excitation processes.

DETERMINING NUCLEAR HYPERFINE POPULATIONS IN THE GROUND ELECTRONIC STATE OF ATOMIC HYDROGEN PRODUCED BY THE 193 NM PHOTOLYSIS OF HBR

A variation of velocity‐aligned Doppler spectroscopy is used to study F=0 and F=1 nuclear hyperfine populations in the ground electronic state of atomic hydrogen as the result of a chemical reaction. In this case, the reaction is the 193 nm photolysis of HBr. Not only can the nuclear hyperfine states be studied, but the method allows unequivocal correlation with the spin–orbit states of the bromine atom, i.e., Br(2P1/2) and Br(2P3/2). For H atoms correlated with Br(2P3/2), the F=0/F=1 is 1.2±0.2. Future directions involving the study of this fundamental chemical product state distribution are also discussed.

193 + 222 nm enhanced photolysis of selectively deuterated 2-chloropropane. A direct investigation of site-specific bond cleavage in the intermediate photofragment (s)

Abstract Selectively deuterated 2-chloropropane (CD 3 CHClCD 3 ) is used to investigate photoinduced, site-specific CH (or CD) bond cleavage. In this study, a two-color photolysis approach is employed to create an intermediate photofragment(s) and then produce a neutral H or D atom from that photofragment(s). On a per site basis, the experimental results suggest that 222 nm photolysis of an intermediate, most likely the isopropyl radical, will preferentially induce carbon-hydrogen bond cleavage at the 2-position — not the lowest energy pathway.

LASER-INDUCED PHOTOFRAGMENTATION OF TRIETHYLALUMINUM : MODELING H-ATOM PRODUCTION

A rate‐equation approach is presented that models H‐atom formation during the pulsed laser photolysis of a triethyl metal compound, the specific case being triethylaluminum excited at 193 nm. An excimer laser initiates the chemistry under collisionless conditions, and H atoms are produced that are detected using two‐photon (121.6+364.7 nm) ionization. Experimentally, the H‐atom intensity is monitored as a function of photolysis laser power. Mechanistically, the primary photodissociation step is postulated to involve cleavage of the metal–carbon bond, thereby producing an ethyl radical. This species can then either: (1) form C2H4 and H directly; or (2) absorb an additional photon and produce an H‐atom photofragment. The rate equations and their solutions allow one to calculate how H‐atom production should vary as a function of photolysis laser power, and the interplay between the two H‐atom production channels is calculated for various absorption cross sections and dissociation rates. A comparison with exp...