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An automated apparatus is presented that allows a user to measure the mass spectrum or velocity map images of photofragments during tuning of the dye-laser radiation wavelength and its second harmonic, which interact with an investigated substance in a molecular beam. The NI LabVIEW environment was used for automation. The apparatus was successfully tested when studying the resonant ionization of oxygen molecules. The automation software makes it possible to extend the possibilities of measuring spectral dependences by creating additional modules without modifying the existing program code.

A broadband near-IR photodetector based on a large-area G12180-250A InGaAs photodiode (Hamamatsu) with a 5-mm-diameter photosensitive area was developed and tested. It is designed for detecting the IR luminescence of singlet oxygen. A circuit for signal amplification is assembled on the basis of broadband low-noise operational amplifiers with the subsequent filtering of a signal with an adjustable passband f varied from 15 to 600 kHz, thus allowing one to obtain a time resolution of better than 1 µs, a sensitivity level of 107–109 V/W, and a noise equivalent power NEP [W/Hz1/2] ≈ 1.4 × 10–14λ–1[µm] f [kHz] in a frequency band of 15–600 kHz.

The one-laser two-color resonance enhanced multiphoton ionization REMPI [(1 + 1′) + 1] and velocity map imaging have been applied to investigate formation of molecular oxygen in excited singlet O2( a 1 Δ g ) and ground O2( X 3 Σ g - ) states in the photodissociation of van der Waals complex isoprene-oxygen C5H8-O2. These molecules were found to appear in the different rotational states with translational energy varied from a value as low as ∼1 meV to a distribution with temperature of about 940 K. The observed traces of electron recoil in the images of photoions reveal participation of several ionization pathways of the resonantly excited intermediate states of O2.

The desorption of excited rubidium (Rb) atoms off the surface of helium (He) nanodroplets is studied in detail using femtosecond time-resolved photoion and photoelectron imaging spectroscopy in combination with quantum wave packet simulations. The good agreement of the measured time-dependent velocity distributions with the simulation when exciting the Rb dopant atoms into the 6p-state supports the pseudo-diatomic model (PDM) for the Rb-He droplet interaction, even on the level of quantum wave packet dynamics. Time-resolved photoelectron spectra reveal the partitioning of excitation energy into the dopant and the droplet degrees of freedom.

Using femtosecond pump-probe ion imaging spectroscopy, we establish the key role of I(+) + I(-) ion-pair (IP) states in the predissociation dynamics of molecular iodine I2 excited to Rydberg states. Two-photon excitation of Rydberg states lying above the lowest IP state dissociation threshold (1st tier) is found to be followed by direct parallel transitions into IP states of the 1st tier asymptotically correlating to a pair of I ions in their lowest states I(+)((3)P2) + I(-)((1)S0), of the 2nd tier correlating to I(+)((3)P0) + I(-)((1)S0), and of the 3rd tier correlating to I(+)((1)D2) + I(-)((1)S0). Predissociation via the 1st tier proceeds presumably with a delay of 1.6-1.7 ps which is close to the vibrational period in the 3rd tier state (3rd tier-mediated process). The 2nd tier IP state is concluded to be the main precursor for predissociation via lower lying Rydberg states proceeding with a characteristic time of 7-8 ps and giving rise to Rydberg atoms I(5s(2)5p(4)6s(1)). The channel generating I((2)P3/2) + I((2)P1/2) atoms with total kinetic energy corresponding to one-photon excitation is found to proceed via a pump - dump mechanism with dramatic change of angular anisotropy of this channel as compared with earlier nanosecond experiments.

The photodissociation of van der Waals complexes of iodine X-I2 (X = I2, C2H4) excited via Charge-Transfer (CT) band has been studied with the velocity map imaging technique. Photodissociation of both complexes gives rise to translationally "hot" molecular iodine I2 via channels differing by kinetic energy and angular distribution of the recoil directions. These measured characteristics together with the analysis of the model potential energy surface for these complexes allow us to infer the back-electron-transfer (BET) in the CT state to be a source of observed photodissociation channels and to make conclusions on the location of conical intersections where the BET process takes place. The BET process is concluded to provide an I2 molecule in the electronic ground state with moderate vibrational excitation as well as X molecule in the electronic excited state. In the case of X = I2, the BET process converts anion I2- of the CT state into the neutral I2 in the repulsive excited electronic state which then dissociates promptly giving rise to a pair of I atoms in the fine states 2P1/2. In the case of C2H4-I2, the C2H4 molecules appear in the triplet T1 electronic state. Conical intersection for corresponding BET process becomes energetically accessible after partial twisting of C2H4+ frame in the excited CT state of complex. The C2H4(T)-I2 complex gives rise to triplet ethylene as well as singlet ethylene via the T-S conversion.

The channel of singlet oxygen O2(1Δg) photogeneration from van der Waals complexes of oxygen X–O2 has been investigated to discriminate between two possible mechanisms based on charge-transfer (CT) or double spin-flip (DSF) transitions. The results obtained in this work for complexes with X = ethylene C2H4, 1,3-butadiene C4H6, deuterated methyl iodide CD3I, benzene C6H6 and water H2O and for those investigated previously indicate the DSF mechanism as a source of singlet oxygen. The formation of O2(1Δg) is observed only when the energy of exciting quantum is sufficient for DSF transition. Universally detected low vibrational excitation of O2(1Δg) arising in the photodissociation of van der Waals complexes X–O2 indicates the DSF mechanism as its source. For complex of ethylene C2H4–O2ab initio calculations of vertical energy ΔEvert for DSF and CT transitions have been carried out. The positive results of singlet oxygen formation from C2H4–O2 can be explained by the DSF but not by the CT mechanism.

The speed and angular distribution of O atoms arising from the photofragmentation of C(5)H(8)-O(2), the isoprene-oxygen van der Waals complex, in the wavelength region of 213-277 nm has been studied with the use of a two-color dissociation-probe method and the velocity map imaging technique. Dramatic enhancement in the O atoms photo-generation cross section in comparison with the photodissociation of individual O(2) molecules has been observed. Velocity map images of these "enhanced" O atoms consisted of five channels, different in their kinetic energy, angular distribution, and wavelength dependence. Three channels are deduced to be due to the one-quantum excitation of the C(5)H(8)-O(2) complex into the perturbed Herzberg III state ((3)Δ(u)) of O(2). This excitation results in the prompt dissociation of the complex giving rise to products C(5)H(8)+O+O when the energy of exciting quantum is higher than the complex photodissociation threshold, which is found to be 41740 ± 200 cm(-1) (239.6±1.2 nm). This last threshold corresponds to the photodissociation giving rise to an unexcited isoprene molecule. The second channel, with threshold shifted to the blue by 1480 ± 280 cm(-1), corresponds to dissociation with formation of rovibrationally excited isoprene. A third channel was observed at wavelengths up to 243 nm with excitation below the upper photodissociation threshold. This channel is attributed to dissociation with the formation of a bound O atom C(5)H(8)-O(2) + hv → C(5)H(8)-O(2)((3)Δ(u)) → C(5)H(8)O + O and/or to dissociation of O(2) with borrowing of the lacking energy from incompletely cooled complex internal degrees of freedom C(5)H(8)*-O(2) + hv → C(5)H(8)*-O(2)((3)Δ(u)) → C(5)H(8) + O + O. The kinetic energy of the O atoms arising in two other observed channels corresponds to O atoms produced by photodissociation of molecular oxygen in the excited a (1)Δ(g) and b (1)Σ(g)(+) singlet states as the precursors. This indicates the formation of singlet oxygen O(2)(a (1)Δ(g)) and O(2)(b (1)Σ(g)(+)) after excitation of the C(5)H(8)-O(2) complex. Cooperative excitation of the complex with a simultaneous change of the spin of both partners (1)X-(3)O(2) + hν → (3)X-(1)O(2) → (3)X + (1)O(2) is suggested as a source of singlet oxygen O(2)(a (1)Δ(g)) and O(2)(b (1)Σ(g)(+)). This cooperative excitation is in agreement with little or no vibrational excitation of O(2)(a (1)Δ(g)), produced from the C(5)H(8)-O(2) complex as studied in the current paper as well as from the C(3)H(6)-O(2) and CH(3)I-O(2) complexes reported in our previous paper [Baklanov et al., J. Chem. Phys. 126, 124316 (2007)]. The formation of O(2)(a (1)Δ(g)) from C(5)H(8)-O(2) was observed at λ(pump) = 213-277 nm with the yield going down towards the long wavelength edge of this interval. This spectral profile is interpreted as the red-side wing of the band of a cooperative transition (1)X-(3)O(2) + hν → (3)X(T(2))-(1)O(2)(a (1)Δ(g)) in the C(5)H(8)-O(2) complex.

The CH(3)I A-state-assisted photofragmentation of the (CH(3)I)(2) van der Waals dimer at 248 nm and nearby wavelengths has been revisited experimentally using the time-of-flight mass spectrometry with supersonic and effusive molecular beams and the "velocity map imaging" technique. The processes underlying the appearance of two main (CH(3)I)(2) cluster-specific features in the mass spectra, namely, I(2)(+) and translationally "hot" I(+) ions, have been studied. Translationally hot I(+) ions with an average kinetic energy of 0.94+/-0.02 eV appear in the one-quantum photodissociation of vibrationally excited I(2)(+)((2)Pi(32,g)) ions (E(vib)=0.45+/-0.11 eV) via a "parallel" photodissociation process with an anisotropy parameter beta=1.55+/-0.03. Comparison of the images of I(+) arising from the photoexcitation of CH(3)I clusters versus those from neutral I(2) shows that "concerted" photodissociation of the ionized (CH(3)I)(2)(+) dimer appears to be the most likely mechanism for the formation of molecular iodine ion I(2)(+), instead of photoionization of neutral molecular iodine.

The photodissociation of (56)FeO was studied by means of the velocity map imaging technique. A molecular beam of iron atoms and iron monoxide molecules was created using an electrical discharge with an iron electrode in a supersonic expansion of molecular oxygen. The ground state iron atom Fe((5)D(4)) and FeO concentrations in the molecular beam have been estimated. The dissociation energy of the FeO X (5)Delta ground electronic state was found to be D(0) (0)(FeO)=4.18+/-0.01 eV. The effective absorption cross section of FeO at 252.39 nm (vac), leading to the Fe((5)D(4))+O((3)P) dissociation channel, is approximately 1.2 x 10(-18) cm(2). A (1+1) resonantly enhanced multiphoton ionization spectrum of (56)FeO in the region 39 550-39 580 cm(-1) with rotational structure has been observed, but not assigned. Angular distributions of Fe((5)D(4)) and Fe((5)D(3)) products for the channel FeO--Fe((5)D(4,3))+O((3)P) have been measured at several points in the 210-260 nm laser light wavelength region. The anisotropy parameter varies strongly with wavelength for both channels.

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Time-resolved dynamics of the photodissociation of molecular oxygen, O(2), via the (3)Sigma(u) (-) ion-pair state have been studied with femtosecond time resolution using a pump-probe scheme in combination with velocity map imaging of the resulting O(+) and O(-) ions. The fourth harmonic of a femtosecond titanium-sapphire (Ti:sapphire) laser (lambda approximately 205 nm) was found to cause three-photon pumping of O(2) to a level at 18.1 eV. The parallel character of the observed O(+) and O(-) images allowed us to conclude that dissociation takes place on the (3)Sigma(u) (-) ion-pair state. The 815 nm fundamental of the Ti:sapphire laser used as probe was found to cause two-photon electron photodetachment starting from the O(2) ion-pair state, giving rise to (O((3)P)+O(+)((4)S)) products. This was revealed by the observed depletion of the yield of the O(-) anion and the appearance of a new O(+) cation signal with a kinetic energy E(transl)(O(+)) dependent on the time delay between the pump and probe lasers. This time-delay dependence of the dissociation dynamics on the ion-pair state has also been simulated, and the experimental and simulated results coincide very well over the experimental delay-time interval from about 130 fs to 20 ps where two- or one-photon photodetachment takes place, corresponding to a change in the R(O(+),O(-)) interatomic distance from 12 to about 900 A. This is one of the first implementations of a depletion scheme in femtosecond pump-probe experiments which could prove to be quite versatile and applicable to many femtosecond time-scale experiments.