Probing the dynamics of highly excited toluene on the fs timescale.

Investigation of the dynamics of toluene-h8 (C6H5CH3), toluene-d8 (C6D5CD3) and toluene-α,α,α-d3 (C6H5CD3) has been performed utilizing the VUV pump-IR probe technique on the fs timescale. Using the 5th harmonic (∼160 nm) of a Ti:sapphire laser as the pump beam, two superimposed electronic states, the valence S3 and the Rydberg 4p, were excited by one-photon absorption, followed by ionization and dissociation induced by the probe beam (800 nm). Analysis of the transient signal of the parent (P(+)) and fragment ions ([P-H](+) or [P-D](+)) implies the existence of two different relaxation processes: (i) from the Rydberg and (ii) from the S3 valence state. Using a rate equation model, the decay times have been determined and comparison between the different isotopologues has been made. Conclusions on the relaxation path, the relative displacements of the potential energy surfaces and the activation energies needed have been drawn from the decay times. The signals corresponding to the fragment ions present a small in amplitude, but nonetheless, unambiguous periodical modulation, which is attributed to out-of-plane bending oscillation, involving also the methyl group. The dynamics of the H- and D-loss channels has been investigated. Especially for the case of toluene-α,α,α-d3, where both channels are in operation, it was found that the ratio of the abundance of H/D-loss dissociation reactions decreases as the pump-probe delay time increases.

Interaction of toluene with two-color asymmetric laser fields: controlling the directional emission of molecular hydrogen fragments.

The interaction of toluene with strong asymmetric two-color laser irradiation of 40 fs duration is studied by means of Time of flight mass spectrometry. Highly energetic H2(+) and H3(+) fragment ions are produced through an isomerization process taking place within transient multiply charged parent ions. Comparative study of deuterium labeled toluene isotopes enables the discrimination between molecular hydrogen fragments formed exclusively within the CH3- part from those that require hydrogen atom exchange between the former and the phenyl moiety. It is demonstrated that by manipulating the relative phase of the ω/2ω field components the selective ionization of oriented toluene molecules can be used as a tool to control the directional emission of the H2(+), H3(+) species.

Selective ionization/dissociation of oriented N2O molecules by asymmetric fs laser field.

We report on the selective ionization of oriented nitrous oxide (N2O) molecules in gas phase by the use of an intense asymmetric two-color ω/2ω 40 fs laser field. By means of a time-of-flight mass spectrometer the induced N2O mass spectra have been recorded as a function of the relative phase of the two-color laser fields. It is found that the applied method facilitates the distinction of different dissociation channels that result in fragments with the same mass and kinetic energy. Thus, the potential of the employed technique for phase control of the molecular excitation for the case of N2O is explored.

Ramsey spectroscopy of bound atomic states with extreme-ultraviolet laser harmonics.

We report the experimental measurement of Ramsey interference fringes in the single-photon excitation to a high-lying bound state of atomic argon by pairs of phase-locked, time-delayed, extreme UV high-order-harmonic pulses at 87 nm. High-visibility Ramsey fringes are observed for delays larger than 100 ps, thus demonstrating a potential resolving power >10(5) at this wavelength.

The ejection of triatomic molecular hydrogen ions H(3) (+) produced by the interaction of benzene molecules with ultrafast laser pulses.

The ejection process of triatomic molecular hydrogen ions produced by the interaction of benzene with ultrafast laser pulses of moderate strong intensity ( approximately 10(14) W/cm(2)) is studied by means of TOF mass spectrometry. The H(3) (+) formation can only take place through the rupture of two C-H bonds and the migration of hydrogen atoms within the molecular structure. The H(3) (+) fragments are released with high kinetic energy (typically 2-8 eV) and at laser intensities >or=10(14) W/cm(2), well above that required for the double ionization of benzene, suggesting that its formation is taking place within multiply charged parent ions. The relative ejection efficiency of H(3) (+) molecular hydrogen ions with respect to the atomic ones is found to be strongly decreasing as a function of the laser intensity and pulse duration (67-25 fs). It is concluded that the H(3) (+) formation is only feasible within parent molecular precursors of relatively low charged states and before significant elongation of their structure takes place, while the higher multiply charged molecular ions preferentially dissociate into H(+) ions. The ejection of H(2) (+) ions is also discussed in comparison to the production of H(3) (+) and H(+) ions. Finally, by recording the mass spectra of two deuterium label isotopes of benzene (1,2-C(6)H(4)D(2), 1,4-C(6)H(4)D(2)) it is verified that the ejection efficiency of some molecular fragments, such as D(2)H(+), DH(+), is dependent on the specific position of hydrogen atoms in the molecular skeleton prior dissociation.

Alignment of ethyl halide molecules (C2H5X, X= I, Br, Cl) induced by strong ps laser irradiation.

The alignment of polyatomic molecules under strong 35 ps laser irradiation is investigated for a broad range of laser intensities (10(13)-10(15) W/cm(2)) using time-of-flight mass spectrometry. The dynamic alignment of the molecules under study (C2H5X, X = I, Br, Cl) is verified in single-pulse experiments by recording the fragments' angular distributions, their dependence on the laser intensity, and also the comparison of the ionic signal of the various fragments recorded for linear and circular polarization. For all cases, the angular distributions of the Coulomb explosion fragments are found to be independent of the laser peak intensity, implying that the molecular alignment is taking place during the rise time of the laser pulses at relatively low intensities (approximately 10(13) W/cm(2)). Moreover, the same result implies that the alignment mechanism is close to the adiabatic limit, albeit the laser pulse duration is much shorter than the characteristic rotational times (1/2B) of the molecules under study. Finally, by comparing the angular distributions of the different molecules, we conclude that the degree of alignment is only weakly dependent on the molecular mass and the moment of inertia under the irradiation conditions applied.

Comparative study of multielectron ionization of alkyl halides induced by picosecond laser irradiation.

The interaction of C2H5X, 1-C3H7X, 1-C4H9X, where X = I, Br, Cl, with strong (1 x 10(13)-1.2 x 10(14) W/cm2) 35 ps laser pulses at 1064 nm is studied by means of time-of-flight mass spectrometry. The multielectron ionization following the C-X bond elongation has been verified for the studied molecules. By combination of the intensity dependence of the ion yields, the estimated kinetic energies of the released fragment ions, and their angular distributions, we have identified the different dissociation channels of the transient multiply charged parent ions. From the dependence on the laser intensity of the ratio of the doubly charged halogen ions to the singly charged ones, it is concluded that the molecular coupling with the laser field increases with the molecular size.

Interaction mechanism of some alkyl iodides with femtosecond laser pulses.

The interaction of 1-iodopropane, 2-iodopropane, 1-iodobutane, 2-iodobutane, and 1-iodopentane with (5 x 10(13-)5 x 10(15) W/cm2) femtosecond laser fields is studied by means of a time-of-flight mass spectrometer. It is found that multiphoton ionization (MPI) and field ionization (FI) processes are involved in the molecular ionization. The contribution of these processes can be distinguished using the peak profile of the ions in the mass spectra. Thus, from the mass spectra of 2-iodoropane and 2-iodobutane, it is concluded that MPI processes are taking place even for Keldysh parameter values gamma approximately 0.3. The field ionization process depends on the characteristics of the molecular binding potential well and leads to an asymmetric charge distribution of the transient multiply charged parent ions. In the case of 1-iodobutane, the MPI processes lead to a stable doubly charged parent ion production with a laser intensity threshold higher than that found for I2+ ions. In addition, the isomers studied exhibit distinct differences in their mass spectra and their origin is discussed in detail.