Subhash Deshmukh

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.

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.

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...

Substantial H-atom production from the 193 nm photolysis of triethylaluminum

Abstract The 193 nm irradiation of triethylaluminum (TEA) produces a significant amount of atomic hydrogen. Doppler profiles at Lyman-α (121.6 nm) reveal that the H atoms have little kinetic energy, the mean kinetic energy being ⩽ 0.35 eV. These observations are interpreted in the context of possible photodissociation pathways. The most likely route involves a single-photon absorption followed by dissociation to form the ethyl radical, which can further dissociate to produce C 2 H 4 and H.

Using lasers to understand and control the chemistry of semiconductor-related precursors

We present results on the laser-induced photochemistry, ablation, and deposition of a variety of precursor compounds used for III-V semiconductor growth. With compounds such as monoethylarsine, triethylarsenic, and triethylgallium, our efforts are focused on using lasers to generate and detect atomic hydrogen, a species that is known to be a good radical scavenger in certain III-V semiconductor growth environments. We also present results on the laser ablation of inorganic salts that may be useful as precursors for III-V thin-film growth. Here, K3Ga3As4 and K2Ga2Sb4 are irradiated with various excimer laser wavelengths, and we report on the ablation and deposition chemistry induced by such radiation. The prognosis for viable film growth using this approach is also discussed. Finally, recent efforts involving the photochemistry and subsequent deposition characteristics of a Pt- containing organometallic compound, C5H5Pt(CH3)3, are presented.

Determining dissociation pathways in the 193 nm photolysis of monoethylamine

Abstract Atomic hydrogen is produced subsequent to the 193 nm photolysis of monoethylamine under collisionless conditions. Experiments utilizing the selectively deuterated compounds C2H5ND2 and C2D5NH2 are used to determine the site(s) from which H (or D) atoms are produced. At low to moderate photolysis power, the dominant route for atomic hydrogen generation involves NH (or ND) bond cleavage. The results of photolysis-power-dependence studies and H-atom Doppler profiles are also used to investigate the overall mechanism for H-atom production.

Controlling H atom production in the 193 nm laser photolysis of triethylarsenic

We report on the production of atomic hydrogen subsequent to the 193 nm photolysis of triethylarsenic (TEAs) using an excimer laser. The H atoms are probed via two‐photon (121.6+364.7 nm) ionization, and the resulting H atom Doppler profile at Lyman‐α is presented. Photolysis power dependence studies demonstrate that substantial H atom formation occurs at relatively low laser powers. However, the H atom signal actually begins to diminish as the photolysis laser power is increased beyond ∼70 MW/cm2. Correlations with time‐of‐fight mass spectral data suggest that ion channels are being accessed. The possible mechanisms for TEAs excitation that lead to H atom formation/depletion are presented, and the implications of these observations on controlling carbon incorporation in the laser‐enhanced growth of films of GaAs, AlGaAs, etc. are discussed.