Bernard L. G. Bakker

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...

Photodissociation of hydrogen iodide in the A-band region 273-288 nm

The photodissociation of hydrogen iodide in the A-band region was investigated between 273 and 288 nm using the photofragment imaging technique. At 17 wavelengths the I* (2P1/2)/I(2P3/2) branching ratio was measured by recording the corresponding hydrogen atomic fragment using multiphoton ionization at 243 nm. The branching ratios are in good agreement with previous measurements and confirm that four (final) states 3Π(0+), 3Π(1), 3Σ(1), and 1Π(1) are involved in the A-band absorption of HI and that those states dissociate adiabatically to form H+I* and H+I following parallel and perpendicular transitions, respectively. The deficiency of the model of three unbound states and that of the δ-approximation is substantiated.

Photophysics of O2 excited by tunable laser radiation around 193 nm

The photodissociation/ionization dynamics of O2 around 193 nm have been studied using a narrowband tunable ArF excimer laser and the velocity map imaging technique. Angular and kinetic energy distributions of the product O+ ions and O(3P2) atoms are recorded and analyzed. The production of O(3P2) atoms is resonance enhanced on the one-photon level by the B 3Σu−(v=4) state, which is part of the B 3Σu−←X 3Σg− Schumann–Runge bands. Angular distribution measurements for individual rotational levels of the B state yield values for the anisotropy parameter, βSR, which are in good agreement with the values predicted by independent measurements of predissociation lifetimes from spectral linewidths. An average value of β=0.48 is found for the underlying Herzberg continuum at 193 nm. O2+ production is enhanced on the two-photon level via members of the nsσg(n−1)dπg1Σg+ Rydberg series terminating at higher vibrational levels of the ion. The high Rydberg states autoionize into the O2+ X 2Πg ground state or absorb one...

The sequential two photon dissociation of NO as a source of aligned N(2D), N(4S) and O(3P) atoms

Abstract Velocity map imaging is used to characterise the angular distributions of atoms which are formed from the sequential two photon excitation of NO via the A 2 Σ + state. Molecules excited at 226 nm and dissociated at 339 nm from the same laser source yield two sets of products, ground state O( 3 P) atoms in conjunction with N( 2 D) and N( 4 S). The angular distributions are found to be well described by β parameters. The use of this method as a source of monoenergetic aligned atoms for studies of reaction dynamics is discussed.

Completely inverted ClO vibrational distribution from OClO(2A2 24,0,0)

The gas phase dissociation dynamics of symmetric chlorine dioxide highly photoexcited into its 2A2 24,0,0 vibronic level has been investigated by monitoring both the kinetic energy release and the angular distribution of the O(3P2) products via velocity map imaging. The resulting O(3P2) kinetic energy profile provides full the vibrational distribution of the formed ClO(X 2Π,v) radicals. The measured O fragment kinetic energy distribution reflects an unprecedented nonstatistical vibrational state population of their diatomic partners, where the excess energy is focused only in the three highest possible ClO vibrational levels v=18–20, with little fragment rotation. The anisotropy of the dissociation process sheds new light on the fragmentation dynamics of this stratospherically relevant triatomic molecule.

Photodissociation dynamics of excited by 193 nm light

Abstract The two-photon dissociation dynamics of 13 C 16 O is studied using a tunable ArF excimer laser and the velocity map imaging technique. C ( 1 D ) atoms formed are detected via (1+1) resonance-enhanced multi-photon ionization (REMPI). The measured C ( 1 D ) yield spectrum is assigned to the a 3 Π(v=2)← X 1 Σ + (v=0) transition. Images containing velocity and angular distribution information are measured for the Q(1), R(0) and R(1) lines. The dissociation energy is confirmed to be 11.09±0.02 eV . Dissociation takes place after one-photon excitation of the a 3 Π 1 (v=2) state via a ΔΩ=0 (parallel) transition. A suggested dissociation process is predissociation of a 3 Π 1 Rydberg state on a 16±5 ps time scale.

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...

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...

Two-photon dissociation of NO near 275 nm investigated by velocity map imaging

The two-photon dissociation of NO at wavelengths near 275 nm has been investigated by velocity map imaging of the 3 .

Crossed-beam velocity map imaging of collisional autoionization processes

Applying the velocity map imaging technique Penning ion formation as well as generation of associative ions is observed in autoionizing collisions of metastable neon atoms (Ne* 2p5 3s 3P2,0) with ground state argon targets in a crossed molecular beam experiment. Metastable neon reactants are obtained by nozzle expansion through a dc discharge ring. The quality of the obtained results clearly demonstrates the suitability of this new, particularly straightforward experimental approach with respect to angle and kinetic energy resolved investigations of Penning processes in crossed-beam studies which are known to provide the highest level of detail.