A. P. Baronavski

Ultraviolet photodissociation dynamics of H2S and D2S

Nascent SH(X 2Πi, v‘=0,1) and SD(X 2Πi, v‘=0,1) rotational state population distributions, spin–orbit state population ratios, and Λ‐doublet state population ratios have been measured following the UV excimer laser photodissociation of H2S (λ=193, 222, and 248 nm) and D2S (λ=193 and 222 nm), respectively. Nascent SH(X 2Πi, v‘=0) rotational state distributions following 193 nm photodissociation of cold H2S in a free jet expansion vs 300 K H2S in a flowing gas cell were essentially the same, indicating that photofragment angular momentum must be originating predominantly in the dissociation event, and not from rotational energy contained in the parent triatom. Laser excitation spectra of SH(X 2Πi, v‘=1) and SD(X 2Πi, v‘=1) have been recorded for the first time. Rotational state distributions for SH(X 2Πi, v‘) and SD(X 2Πi, v‘) are independent of v‘. Λ‐doublet population ratios of the nascent photofragments are essentially unity for all the cases measured. The nascent rotational state distributions are consi...

ULTRAFAST PHOTODISSOCIATION DYNAMICS OF THE S1 AND S2 STATES OF ACETONE

The photodissociation dynamics for the two lowest excited electronic states (S1 valence state and the S2, 3s Rydberg state) of acetone (h6 and d6) have been studied using femtosecond mass-resolved photoionization spectroscopy. The S1 state dynamics was investigated by near ultraviolet (UV) pump (∼265 nm) and deep UV (205 nm) or visible (410 nm) probe. The primary dissociation time is instrument-limited, providing a 200 fs upper limit to the lifetime. The acetyl ion signal exhibits a subpicosecond decay and a persistent signal. The fast decay is consistent with results from Kim et al. [J. Chem. Phys. 103, 477 (1995)] for two-photon excitation to near the 4s state. The persistent signal is due to probe-induced ionization of acetyl radicals that are stable with respect to secondary dissociation. The S2 excited state lifetime measured for acetone-d6 using 194 pump and 259 nm probe is 13.5±1.0 ps. This is almost three times longer than we previously determined for this state in acetone-h6, 4.7±0.2 ps. The seco...

Vibrational energy relaxation of aqueous azide ion confined in reverse micelles

Vibrational energy relaxation (VER) times have been measured by ultrafast infrared spectroscopy near 2000 cm−1 for the antisymmetric stretching ν3 band of azide ion in water pools of nonionic reverse micelles (RM). The water pool radii were varied in the 1–3 nm range by adjusting the water-to-surfactant molar ratio, ω=[H2O]/[surfactant]. Compared to the value measured in this work for bulk water (0.81±0.06 ps), the VER decay times are about three times longer (2.5±0.2 ps) for the smallest RM studied (ω=1) and become shorter with increasing ω and RM size but do not reach the bulk value at the largest ω studied. Solvent shifts of the azide vibrational band in RMs have been previously reported [Langmuir 18, 7401 (2002)], and in a manner similar to the VER rates, tend toward the bulk water value with increasing ω. Studies of the VER dynamics of azide ion in RMs are used to investigate confinement effects on solvation and to explore the effects of continuously modifying the solute–solvent interaction by varyin...

Reorientation and vibrational energy relaxation of pseudohalide ions confined in reverse micelle water pools

Reorientation and vibrational energy relaxation times have been measured by ultrafast transient polarization IR spectroscopy for the antisymmetric stretching band in the 2000–2200 cm−1 region of pseudohalide ions, N3−, NCO−, and NCS−, confined in nanosize water pools of reverse micelles (RMs). The RMs are composed of nonionic nonylphenyl poly-oxyethylene surfactant in cyclohexane. Vibrational energy relaxation times (T1) of the NCO− and NCS− ions are about three times longer in small RMs compared to bulk water, similar to our earlier results on the N3− ion [J. Chem. Phys. 118, 7074 (2003)]. The longer T1 times are attributed to the reduced interaction between the ions and water molecules due to confinement effects and hydration of the surfactant headgroups by water molecules. Reorientation times (TR) of the N3− and NCS− ions are found to be more than one order of magnitude longer in small RMs than in bulk H2O. The observed longer TR times support the notion that water molecules inside RMs have very restri...

Ultrafast photodissociation studies of acetyl cyanide and acetic acid and unimolecular decomposition rates of the acetyl radical products

Unimolecular decomposition rates for acetyl radical following the photodissociation of acetyl cyanide and acetic acid near 193 nm have been studied using ultrafast mass-resolved photoionization spectroscopy. In both cases, the parent decays with an instrumentally limited lifetime, while the acetyl radical behaves in a manner consistent with an RRKM mechanism, in contrast to our previous results on acetone. It is necessary to convolute the population distribution with the microcanonical RRKM rates in order to achieve this agreement. We have also undertaken an ab initio study of the excited states of acetyl cyanide to clarify the assignments of these states. The state excited at 193 nm arises from a π→π* transition with a calculated transition velocity dipole moment oriented at an angle of 57° with respect to the C–C≡N bond, resulting in an anisotropy parameter of −0.22. This is in reasonable agreement with the previous data of North et al. [J. Phys. Chem. A 101, 9224 (1997)]. The apparent RRKM behavior of ...

Ultrafast studies of the photodissociation of the acetone 3s Rydberg state at 195 nm: Formation and unimolecular dissociation of the acetyl radical

Ultrafast deep UV mass-resolved photoionization spectroscopy has been used to investigate the photodissociation dynamics of the 3s Rydberg state of acetone. Single photon excitation at 193–195 nm is followed by single photon (at 260 nm) and two photon (at 390 nm) ionization and the signal is measured for both the acetone and acetyl photoions. The acetone Rydberg state lifetime determined from both single and two photon detection is surprisingly long, 4.7±0.2 ps. The higher probe energy for two photon ionization results in a lower minimum acetyl internal energy for ionization, so that part of the measured signal is due to neutral acetyl dissociation dynamics rather than only dissociative ionization of excited state acetone (which is the case for single photon ionization at 260 nm). The secondary dissociation rate of the neutral acetyl intermediate is measured, clearly establishing that photodissociation via the first Rydberg state of acetone occurs by a sequential dissociation mechanism. The acetyl dissoci...

Application of saturation spectroscopy for measurement of atomic Na and MgO in acetylene flames

The technique of laser induced fluorescence saturation spectroscopy is evaluated for determination of absolute atomic and molecular concentrations in flame sources. The combustor is an aspirating slot burner using premixed acetylene and air to create a stable homogeneous flame source. Concentration measurements of atomic sodium and MgO (X 1Σ+ and A 1Π) are made by saturation spectroscopy and laser induced fluorescence techniques. Independent atomic and molecular absorption measurements are made for Na and MgO (X 1Σ+) which are in agreement with saturation spectroscopy concentration determinations. The effect of using various laser beam profiles (rectangular, Gaussian, and truncated Gaussian) are evaluated. It is found that MgO exists in the flame in a significant inversion in the (A 1π) state and possible production mechanisms for MgO in the flame are considered.

Production of OH from the collision‐free photodissociation of nitromethane at 266 nm

Photodissociation of nitromethane, CH3NO2, at 266 nm under collision‐free conditions results in the single photon production of electronic ground state OH(X 2Πi). A rotational ‘‘temperature’’ of 1750±50 K describes the rotational distribution of the nascent OH radical. A quantum yield of 0.004±0.001 is determined for the photolytic pathway producing the OH radical. Within experimental uncertainty, no Λ‐doublet population preference was observed for OH production from H2O2, HNO3, or CH3NO2 photolysis. If a saturation correction is not utilized apparent Λ‐doublet population differences are found. This incorrect saturation compensation may account for previous literature Λ‐doublet population ratios, fi/ f′i, that are greater than one.

Vibronic dependence of the B̃ state lifetimes of CH3I and CD3I using femtosecond photoionization spectroscopy

Tunable deep UV femtosecond photoionization spectroscopy with single photon excitation and wavelengths longer than 192 nm has been used to determine predissociation-mediated excited state lifetimes for many vibronic levels of the B (6s[2] Rydberg) state of CH3I and CD3I. These include states with vibrational excitation in the ν1, ν2, ν3, and ν6 modes. We have previously reported lifetime measurements for the origin bands [Chem. Phys. Lett. 222, 335 (1994)]. The vibronic and isotopic dependences presented here qualitatively agree with various aspects of results from two indirect measurements. Our results corroborate the counterintuitive result from the resonance Raman work by Wang and Ziegler [J. Chem. Phys. 95, 288 (1991)] that the level singly excited in the C–I stretching mode (31) dissociates more slowly (we measure ∼4.0 ps for both CH3I and CD3I) than the vibrationless levels (1.38 and 1.90 ps, respectively). In contrast to the resonance Raman results and similar to those from resonance enhanced mult...

Optical properties and dynamics of a photochromic bisthienylethene in solution and in a polymer film

Abstract Optical switching dynamics and fatigue were investigated for a photochromic molecular switch, bis(5-pyridyl-2-methylthien-3-yl)perfluorocyclopentene (PMTFC). Femtosecond transient absorption was measured using 270 nm pump and broadband visible probe pulses for samples in ethanol solution and as a dispersion film of poly(butyl acrylate-co-acrylic acid). The optical switching occurs in ≤3 ps in both cases. The spectral evolution observed in solution within 20 ps is absent in the film, suggesting that the switching dynamics are sensitive to the molecular environment. PMTFC in the copolymer film demonstrated evidence of fatigue after ∼1500 complete cycles of optical switching.