Gregory E. Hall

Primary and secondary processes in the 193 nm photodissociation of vinyl chloride

We have investigated the photodissociation of vinyl chloride (H2CCHCl) at 193 nm using the technique of photofragment translational spectroscopy. The experiments were performed at the Chemical Dynamics Beamline at the Advanced Light Source and used vacuum ultraviolet synchrotron radiation for product photoionization. We have observed five primary dissociation channels following an initial π*←π excitation. The majority of Cl atoms originate from an excited-state dissociation. The remaining dissociation channels are consistent with competition on the ground electronic state following internal conversion from the optically prepared state. These channels include atomic and molecular hydrogen elimination, HCl elimination, and a translationally slow Cl elimination channel. We have also identified and characterized two secondary decomposition channels: (1) the elimination of Cl from chlorovinyl radicals following the primary atomic hydrogen elimination channel, and (2) hydrogen atom elimination from vinyl radica...

Reactions of velocity‐aligned atoms probed by Doppler profiles: H+O2→OH+O

Final state resolved vector properties of the reaction H (2S)+O2(3Σ−g)→OH(2Π)+O(3P) have been measured using Doppler spectroscopy. Velocity‐aligned H atoms were generated by photodissociation of H2S with linearly polarized ArF excimer laser light. The correlated velocity and angular momentum distribution of nascent OH reaction products was probed by laser induced fluorescence. Techniques are described for analyzing the line shapes, modeling the bipolar moments of the OH velocity and angular momentum distribution as parametric functions of the scattering angle. Distinctive Doppler‐broadened lineshapes indicate scattering angle distributions with a strong forward and a weaker backward maximum. A qualitatively different angular momentum polarization for the two lambda doublet components of the OH product is required to explain the Doppler profiles.

LINE SHAPE ANALYSIS OF DOPPLER BROADENED FREQUENCY-MODULATED LINE SPECTRA

We present a method for recovering Doppler broadened absorption line shapes from frequency modulated (FM) line spectra. The method of analysis is calibrated and demonstrated with thermalized CN radicals produced by photodissociation of cyanogen (NCCN), probed on the A–X system near 800 nm with a frequency modulated Ti: sapphire ring laser. Nonthermal, Doppler broadened lines from translationally nascent photofragments can also be recovered by direct transformations of experimental FM line profiles acquired with a time resolution exceeding 100 ns. The superior signal‐to‐noise afforded by FM spectroscopy, relative to other direct absorption methods, should encourage the application of transient FM spectroscopy to problems in photoinitiated reaction dynamics.

Vector signatures of adiabatic and diabatic dynamics in the photodissociation of ICN

Nascent Doppler profiles of CN (X 2∑+) fragments from the A band photodissociation of room temperature ICN have been measured using high-resolution transient frequency modulated absorption spectroscopy. Results for dissociation at 222 nm, 248 nm, 266 nm, and 308 nm are presented. From the Doppler profiles of multiple CN states, we determine branching ratios of the coincident atomic iodine states, and bipolar moments characterizing the CN velocity and angular momentum anisotropy. The measurements provide sensitive tests of the strengths of optical coupling to the excited states contributing to the A band continuum, and the adiabatic and diabatic dynamics leading to the observed product states. Precise velocity measurements resolve differences in the average energy of the ICN molecules leading to selected fragment channels. We find a bond energy for ICN of 26 980±100 cm−1, somewhat higher than previous literature values.

Time‐resolved frequency modulation spectroscopy of photochemical transients

We report the use of time‐resolved frequency modulation (FM) spectroscopy for the measurement of photochemically generated radicals. CN radicals from the 193 nm photodissociation of cyanogen (NCCN) have been detected using a phase‐modulated cw Ti:sapphire ring laser, probing single rotational lines of the A 2Π←X 2Σ system. The combination of sensitivity with time and frequency resolution is more than adequate to record Doppler‐broadened line shapes of collisionless photofragments. Significant signal‐to‐noise enhancement is demonstrated compared to a dual‐beam transient absorption technique.

Photodissociation of acetone at 193 nm: Rotational‐ and vibrational‐state distributions of methyl fragments by diode laser absorption/gain spectroscopy

Diode laser transient absorption/gain spectroscopy is used to monitor time‐dependent populations of CD3 fragments formed in the photodissociation of acetone‐d6 at 193 nm. Selected rotational lines have been measured in the ν2 ‘‘umbrella’’ fundamental and first two hot bands, and in the ν3 asymmetric stretching fundamental band. Substantial growth is observed in the vibrationless state on the time scale of vibrational relaxation. We estimate that only about 15% of the nascent CD3 population is formed in the vibrational states we detect: ν2=1 and 2, ν3=1, and the vibrationless state. Most of the nascent methyl population is evidently spread among many undetected vibrational states. These results complement previous measurements of acetone photofragments by infrared emission, multiphoton ionization, and laser‐induced fluorescence. Our inferred global vibrational distribution is consistent with a two‐step fragmentation.

Resonance enhanced multiphoton ionization time-of-flight study of CF2I2 photodissociation

The resonance enhanced multiphoton ionization time-of-flight (REMPI-TOF) technique was applied to study the complex dissociation of CF2I2 in a supersonic expansion. Using nanosecond excitation at 248, 266, and 304 nm and probing the I3/2 and/or I1/2 photoproducts, we determined the photofragment speed distribution, the recoil anisotropy parameter β and the branching ratio for each of the decay channels. At 248 nm the dissociation proceeds according to a concerted three-body decay, CF2I2→hvCF2+I1/2+I3/2 (90%) or CF2I2→hvCF2+I3/2+I3/2 (10%) consistent with our previous results obtained from photofragment translational spectroscopy. At 304 nm three competing decay channels were found to be operative. In decreasing order of branching, these are: two-body decay to CF2I +I1/2, sequential three-body decay CF2I2→ lim hvCF2I + I3/2→CF2 + I3/2+I3/2, and a concerted three-body decay yielding CF2 +I3/2+I3/2. The positive, almost maximum β values observed for all the primary steps in the 248, 266 and 304 nm photodisso...

Near‐infrared vibronic spectrum of the CH2 b̃ 1B1←ã 1A1 transition

A Doppler‐limited high resolution vibronic spectrum of the methylene (CH2) b 1B1←a 1A1 transition in the near‐infrared wavelength region has been obtained using transient absorption techniques. The radical was produced by 308 nm excimer laser photolysis of ketene (CH2CO) in a flow system. The analysis of this spectrum confirms the reassignments of some previously observed bands as well as the presence of new vibronic bands predicted by a recent ab initio calculation [Green et al., J. Chem. Phys. 94, 118 (1991)]. We also measured the Doppler broadened profiles of low‐J rotational lines of CH2 under collisionless conditions. From the line profile analysis, we find that the 308 nm photolysis of ketene produces fragments with isotropic and uncorrelated velocity and angular momentum distributions. The Doppler profiles also provide a view of the coincident product state distributions. The measured Doppler profiles are consistent with a rotational distribution of CO produced in coincidence with low energy CH2 ...

Dissociation of CD3I at 248 nm studied by diode laser absorption spectroscopy

Transient diode laser absorption spectroscopy has been used to monitor the rotational and vibrational populations of CD3 radicals following photodissociation of CD3I by 248 nm light. Rotational lines in the ν2=1–0, 2–1, 3–2, and 4–3 bands have been analyzed to give time‐dependent CD3 populations following photodissociation of 100 mTorr samples of neat CD3I. Prompt absorption signals are small, but positive for all observed rotational lines of all observed bands, providing clear evidence against population inversion in the low levels of the ν2 vibration of methyl photoproducts. The vibrational population distribution in ν2 a few μs after photodissociation is 4:3:2 for v=0:1:2 with about 20% uncertainties. A strong variation in time dependence with rotational level is consistent with an initially hot rotational distribution (≥2000 K) that cools on the μs time scale of methyl translational moderation in the bath of undissociated methyl iodide. We believe the origin of the rotational excitation to be translat...

Relationship between bipolar moments and molecule-frame polarization parameters in Doppler photofragment spectroscopy

In terms of the molecular-frame polarization parameters aq(k)(p), an equation is derived that describes the shape of a photofragment Doppler profile as a function of the three angles Γ, Δ, and Φ that specify the photolysis and probe laser polarizations about the detection axis. This expression is specialized to linearly polarized photolysis and probe laser beams. For the particular value of the angle between the probe laser polarization and the detection axis, Δ=π/2, this equation can be reduced to the form of well-known laboratory-frame expressions that use the bipolar moment formalism introduced by Dixon. Comparison of these forms shows the equivalence of the two formalisms and gives the relationships between the bipolar moments βQK(k1k2) and the molecule-frame aq(k)(p) parameters. We show that linear combinations of the bipolar moments completely describe photofragment polarization in the molecular frame and possess distinct quantum mechanical significance. In particular, it is shown that the coherent ...