D. Proch

Controlling molecular ground-state dissociation by optimizing vibrational ladder climbing

To achieve large population transfer to high vibrational levels in a selected ground-state mode of a polyatomic molecule [Cr(CO)6], we apply chirped femtosecond mid-infrared laser pulses at 2000 cm−1 to optimize vibrational ladder climbing as an energy deposition mechanism, which in turn controls the outcome of a unimolecular dissociation process. Its dependence on excitation parameters (frequency, intensity, chirp) is investigated and found to be in excellent agreement with a theoretical calculation. In particular, it is shown that optimizing vibrational ladder climbing allows for coherently controlled excitation even in a polyatomic molecule.

Molecular dissociation by mid-IR femtosecond pulses

Abstract By focusing a MIR femtosecond laser in a cell containing gas-phase metal carbonyls, the resonant infrared multiphoton dissociation of molecules was observed. Cr(CO)6,Mo(CO)6,W(CO)6, and Fe(CO)5 could easily be dissociated, which requires an excitation to at least v=7 or 8 of the CO stretch vibration. After irradiation with ∼150 fs pulses at 5 μm the metal carbonyl practically disappears in favor of free CO, as detected by the IR spectrum. By comparing the power dependence of the total conversion with a model, we can infer that only few vibrational degrees of freedom are involved in the excitation process.

Getting ahead of IVR: A demonstration of mid-infrared induced molecular dissociation on a sub-statistical time scale

Gaseous diazomethane (CH2N2) has been irradiated with femtosecond laser pulses tuned to the CNN asymmetric stretch at 2100 cm−1 in the mid-infrared. Laser-induced fluorescence detection of 1CH2 [537 nm, b1B1(0 16 0)←a1A1(0 0 0)] confirms infrared (IR) multiphoton-induced scission of the C–N bond on two distinct time scales, 480±70 fs and 36±8 ps. The longer time scale is consistent with a statistical dissociation process; the shorter one is indicative of directed dissociation, which occurs more rapidly than statistical intramolecular vibrational energy redistribution because of direct coupling of the vibrational energy from the IR-excitation mode into the reaction coordinate. Anisotropy measurements demonstrate that the CH2 group bends significantly out of the molecular plane prior to the dissociation.

Programmable amplitude- and phase-modulated femtosecond laser pulses in the mid-infrared.

We present a scheme to produce programmable phase- and amplitude-modulated femtosecond laser pulses in the mid-infrared regime of 3-10mum by difference frequency mixing. The 80-fs signal output of an optical parametric amplifier is shaped with a liquid-crystal mask and mixed in an AgGaS(2) crystal with a temporally stretched idler pulse. Without changing the mechanical alignment, we produce programmable amplitude modulations and chirped pulses at lambda=3mum with energy as high has thas 1muJ . This scheme, further, allows the generation of controllable pulse sequences. The results are in good agreement with theoretical simulations.

Efficient tunable ultraviolet source based on stimulated Raman scattering of an excimer-pumped dye laser.

We report on the design and performance of a tunable ultraviolet source based on stimulated Raman scattering in compressed hydrogen. The 440-nm output of an excimer-pumped dye laser was employed as pump source. By lowering the gas temperature to 78 K, an improvement of ~70% was obtained in the efficiency of conversion to the seventh anti-Stokes order at 193 nm.

Highly efficient, high-quality phase-conjugate reflection at 308 nm using stimulated Brillouin scattering.

We report efficient, diffraction-limited phase conjugation of a XeCl laser beam (308 nm) using stimulated Brillouin scattering. Reflectivities approaching 100% are obtained by focusing a XeCl laser (<0.15-cm(-1) linewidth) at 300 GW/cm(2) into heptane or ethanol.

Rovibronic energy transfer from N2(a 1πg) to CO(A 1π) studied by laser REMPI spectroscopy

The collision‐induced electronic energy transfer N2(a 1πg,v′) +CO(X 1Σ+,v″=0) →N2(X 1Σ+g,v″) +CO(A 1πg,v′)+ΔE is studied in a gas cell. N2(a 1πg,v′, J′) is prepared by two‐photon (2hν1) absorption from the ground state. CO(A 1πg,v′, J′) is probed by two‐photon (hν1+hν2) ionization via CO(B 1Σ+) as the resonant intermediate state. Experiments show that the overall energy transfer cross sections exceed that of gas kinetic collisions by a factor of 3–4. The energy mismatch ΔE is the determining factor controlling the branching ratio from one N2(1πg,v′) donor to different vibrational levels of CO(A 1π,v′). For small values of ΔE, CO(A 1πg,v′, J′) shows a Boltzmann‐like rotational level population. Its rotational temperature scales with ΔE. About 28% of the excess energy funnels into the rotation of CO(A 1π). An explanation for the observed rotational distribution of CO* and the energy transfer mechanism is given. The rate constants are analyzed in terms of the surprisal.

State-selective ionization of nitrogen by resonance-enhanced three- and four-photon excitation

Abstract Several efficient approaches for the generation of vibrationally cold nitrogen ions by resonance-enhanced multi-photon ionization are examined. More than 10 5 ions per laser pulse are demonstrated for single-laser, 2+1 photon ionization via a 1 Π g (ν′ = 10) which accesses only the ν + = 0 level of the ion. The rotational distribution of the photoions is analysed by laser-induced fluorescence under molecular beam conditions. A Δ N = 0 rotational propensity rule is found for the ionizing step when ionizing out of the Π + component of the a 1 Π g state. In addition, two-laser 2+1+1 photon ionization with a 1 Π g and c′ 4 1 Σ u + or c 3 1 Π u vibrational levels as the intermediates is investigated. With the excitation of a(ν′ = 4) and c′ 4 , c 3 (ν = 0) strong ion signal is detected for an excess energy above the ionization threshold of about 16400 cm −1 , which yields X 2 Σ g + (ν + = 0) as well as A 2 Π u ions, but no signal at about 7500 cm −1 which would confine the ionic state distribution to X 2 Σ g + (ν + = 0). Ionization via a (ν′ = 1) and c 3 (ν = 0) is investigated as an alternative scheme to produce ν + = 0 ions. Though not being as efficient as the 2+1+1 photon ionization involving some of the c′ n (ν) states this excitation scheme yields 5×10 4 ions per laser pulse. A first experiment to prepare N 2 + in the ν + = 1 level is carried out.

Rotational transitions of N2(a 1Πg) induced by collisions with Ar/He and N2(a 1Πg)–N2(X 1Σ+g) rovibronic energy transfer studied by laser REMPI spectroscopy

This paper reports the results of two related experiments: (A) The state‐to‐state rotational transition probabilities of N2(a 1Πg) in collisions with rare gas atoms (Ar or He) were measured by the technique of two‐step multiphoton ionization. Results show that the selection rule antisymmetric–symmetric is obeyed. The transition probability drops rapidly with increasing ‖ΔJ‖. A propensity rule related to the Π+ or Π− symmetry conservation of the electronic wave function during the collision induced rotational transition holds. (B) The cross section for the rovibronic energy transfer between N2(1Πg) and N2(X 1Σ+g) is found to be ∼28 A2. N2(a 1Πg) product populations show a Boltzmann‐like distribution with a rotational temperature suggesting an equipartition of the available energy among the rotational and translational degrees of freedom of the products. A mechanism invoking an intermediate collision complex along with intermolecular electron exchange may explain the results.

Photoelectron angular distributions and vibrational branching ratios of CO (2+1)‐photon ionization via the B 1Σ+ state

We present vibrational branching ratios and photoelectron angular distributions for resonantly enhanced (2+1)‐photon ionization of CO. The excitation ladder involves the B 1Σ+(vi=0 or 1) Rydberg state. Contrary to expectations fostered by the Franck–Condon principle, ionization via vi=0 branches into vibrational states v+=0–4. Such phenomena are also observed in the case of vi=1, but only to a minor extent. The angular emission patterns of the photoelectrons ejected during the ionizing step are of distinct character in that they are highly anisotropic for Δv=vi−v+=0 processes, but show isotropy when due to Δv≠0 transitions. The photoelectron angular distribution which accompanies the Δv=0 ionization of B(vi=1) shows p‐wave character, and hence we may postulate a spherical potential of the Rydberg ion core. The same approximation should hold for the vi=0 state. The dissimilar appearance of the angular distributions when ionizing from this level invites the hypothesis of two individually different ionizatio...