Peter C. Samartzis

Photodissociation study of CH3Br in the first continuum

The photolysis of CH3Br is studied in the energy region of the A band between 4.94 and 5.76 eV using ion imaging. Velocity distributions for both the bromine-atom and methyl-radical photofragments are determined. Our results indicate that transitions to the 3Q0 and 3Q1 states dominate the absorption cross section and the partial cross section to each state is determined. The [Br*]/[Br] branching ratio is found to be strongly dependent on the excitation energy varying between 0.6 and 1.5. Both the bromine-atom and the methyl-radical translational energy distributions suggest that the vibrational distribution in the nascent CH3 is nonstatistical with appreciable excitation in the v2 umbrella mode. The lifetime of the A band is estimated at τ=120±40 fs.

Spin-orbit branching ratios for the Cl atom photofragments following the excitation of Cl2 from 310 to 470 nm

Velocity distributions for the Cl(2P3/2) and Cl(2P1/2) photofragments produced by photolysis of Cl2 in the region between 310 and 470 nm are measured using photofragment velocity mapping. Our results indicate that at short wavelengths the absorption spectrum is dominated by the 1u(1Πu) excited electronic state which produces two ground state chlorine atoms. The 0u+(B 3Πu) state which produces a spin-orbit excited and a ground state chlorine atom becomes significant at 350 nm and dominates the spectrum beyond 400 nm. Analysis of the photofragment angular distributions indicates that the Cl(2P3/2) photofragments are aligned and the magnitude of the alignment is quantitatively determined. Nonadiabatic curve crossing between the 1u(1Πu) and the 0u+(B 3Πu) electronic states is observed and quantified below 370 nm. The measured nonadiabatic transition probability is modeled using the Landau–Zener formula and the position of the curve crossing is estimated at ∼3 eV above the zero-point of ground electronic state...

Photofragmentation study of Cl2 using ion imaging

The Cl(2P3/2) and Cl(2P1/2) photofragments produced by the photolysis of Cl2 at ∼3.5 eV, are detected using (3+1) and (2+1) resonance enhanced multiphoton ionization and their velocity distributions are measured using ion imaging. The measured photofragment translational energy distributions yield a bond dissociation energy of D0=2.474±0.020 eV for ground electronic state Cl2(X1Σg). The angular distributions yield anisotropy parameters of β(2P3/2)=−0.87±0.05 and β(2P1/2)= 1.78±0.05 irrespective of the detection process. The branching ratio [Cl(2P1/2)]/[Cl(2P3/2)] is estimated at 8±1%.

The photodissociation of CH3SCH3 and CD3SCD3 at 220–231 nm investigated by velocity map ion imaging

The photodissociation of the two isotopomers of dimethyl sulfide, CH3SCH3 and CD3SCD3, through the first electronic absorption band at wavelengths 220–231 nm has been studied employing velocity map ion imaging to detect the methyl products. Translational energy and recoil angle distributions have been determined for the CH3 and CD3 fragments either in the vibrational ground state or without product state selection. The measurements indicate that the S–C bond photolysis yields strongly anisotropic product scattering distributions characterized by β parameters within the range (−0.4, −1.0). The β values closest to the limiting value of −1.0 (corresponding to a dipole perpendicular transition) are observed for the methyl fragments produced in the vibrational ground state. The product recoil energy distributions are centered at ≈1.5–1.7 eV (i.e., 65%–75% of the available energy) and are quite narrow (full width at half maximum, FWHM≈0.3–0.5 eV), which indicates that methyl and methylthio fragments are born in...

Nonresonant photofragmentation/ionization dynamics of O2 using picosecond and femtosecond laser pulses at 248 nm

Photodissociation/ionization of molecular oxygen in a cold molecular beam is studied using a short-pulse laser beam at 248 nm and velocity map imaging. Both photoelectron and O+ images are recorded for laser pulsewidths of 5 and 0.5 ps. Most of the observed ionization dynamics takes place after absorption of four laser photons, equivalent to 20 eV excitation energy, in an above threshold ionization process. Two main channels are identified: postdissociative ionization creating O(3P)+O**3s″(1P, 3P) atoms where O** is an electronically excited autoionizing atom, and molecular (auto)ionization to create a range of highly vibrationally excited ground electronic state O2+ ions. The observed O+ signals then arise from resonance-enhanced two-photon dissociation of O2+ or autoionization of O** atoms, while the electron signals arise from ionization of O2 or autoionization of O**. The latter channel can be used to directly scale the photoelectron and O+ signal strengths. The O+ images show strong differences for 0...

Stereodynamics of the photodissociation of nitromethane at 193 nm: unravelling the dissociation mechanism.

The photodissociation of nitromethane at 193 nm is reviewed in terms of new stereodynamical information provided by the measurement of the first four Dixons bipolar moments, β0(2)(20), β0(0)(22), β0(2)(02), and β0(2)(22), using slice imaging. The measured speed-dependent β0(2)(20) (directly related with the spatial anisotropy parameter β) indicates that after one-photon absorption to the S3(2 (1)A″) state by an allowed perpendicular transition, two reaction pathways can compete with similar probability, a direct dissociation process yielding ground-state CH3 and NO2(1 (2)A2) radicals and a indirect dissociation through conical intersections in which NO2 radicals are formed in lower-lying electronic states. A particularly important result from our measurements is that the low recoil energy part of the methyl fragment translational energy distribution presents a contribution with parallel character, irrespective of the experimental conditions employed, that we attribute to parent cluster dissociation. Moreover, the positive values found for the β0(0)(22) bipolar moment indicates some propensity for the fragments recoil velocity and angular momentum vectors to be parallel.

Photodissociation-ionization dynamics of molecular chlorine Rydberg states using velocity map imaging

Velocity map images are reported for photoelectrons and atomic chlorine ions produced during one-color REMPI (resonance enhanced multiphoton ionization) of molecular chlorine via the v=0–15 vibrational levels of the 2 1Πg(4s) Rydberg state. Previous magnetic bottle photoelectron studies by Koenders et al. of the same process have shown that REMPI proceeds by two-photon resonant excitation, followed by core-excitation to a super-excited Rydberg state at the three-photon level. This state undergoes (auto)-ionization, resulting in the formation of ground-state molecular chlorine ions, and/or dissociation to neutral products, resulting in the production of electronically excited neutral chlorine atoms. Photoelectrons arise from ionization of Cl2 and electronically excited Cl atoms, while Cl+ ions arise from the dissociation of Cl2+ and from ionization of the excited Cl atoms. The chlorine ion velocity map images reveal new information on the dissociation–ionization dynamics of superexcited Cl2 and the dissoci...

Multiphoton dynamics of H2 with 248 nm picosecond and femtosecond pulses

Nonresonant excitation of H2 at 248 nm using 5 ps and 0.5 ps laser pulses with intensities 5×1011 and 5×1012 W/cm2, respectively, is studied by measurement of the angle-speed distributions of the H+ photofragment using velocity map imaging. Three main H+ production channels are observed: (a) three-photon dissociation to H(n=1)+H(n=2) followed by photoionization of H(n=2); (b) dissociative (auto)ionization following four-photon absorption to form H(n=1)+H++e−; and (c) autoionization from a doubly excited state producing a H2+ vibrational distribution peaking at (v+=2,7,13) and subsequent photodissociation of these vibrationally excited ions. In the neighborhood of the energy sum of three 248 nm photons lie transitions to the B′ 1Σu+(3pσ) vibrational continuum and the two bound states B″ 1Σu+(4pσ)(v=2) and D 1Πu+(3pπ)(v=4). These states are believed to play an important role in three-photon absorption and at the subsequent fourth photon level. Their effect on the observed kinetic energy and angular distribu...