The quantitative determination of laser-induced molecular axis alignment.

Experiments in the gas phase usually involve averaging observables over a random molecular axis alignment distribution. This deleterious averaging limits insights gained by probes of molecular dynamics, but can be overcome by prealigning molecular axes using laser-alignment methods. However, the transformation from the laboratory frame to the molecular frame of reference requires quantitative knowledge of the axis alignment distribution. The latter is often hard to obtain directly from experimental data, particularly for polyatomic molecules. Here we describe a general maximum-likelihood classification procedure for non-adiabatic numerical alignment simulations with free parameters that employs experimental data from an alignment-dependent probe. This method delivers (i) the most probable molecular frame angular dependence of the probe, and (ii) the most likely laboratory frame axis alignment distribution of the sample, each with a confidence interval. This procedure was recently used for studies of angle- and channel-resolved strong field ionization of 1,3-butadiene in the molecular frame [Mikosch et al., Phys. Rev. Lett. 110, 023004 (2013)], used here as an illustrative example.

Stark-selected beam of ground-state OCS molecules characterized by revivals of impulsive alignment.

We make use of an inhomogeneous electrostatic dipole field to impart a quantum-state-dependent deflection to a pulsed beam of OCS molecules, and show that those molecules residing in the absolute ground state, X(1)Σ(+), |00(0)0>, J = 0, can be separated out by selecting the most deflected part of the molecular beam. Past the deflector, we irradiate the molecular beam by a linearly polarized pulsed nonresonant laser beam that impulsively aligns the OCS molecules. Their alignment, monitored via velocity-map imaging, is measured as a function of time, and the time dependence of the alignment is used to determine the quantum state composition of the beam. We find significant enhancements of the alignment ( = 0.84) and of state purity (>92%) for a state-selected, deflected beam compared with an undeflected beam.

Probing the lifetimes of internally excited amyl nitrite cations.

The photoelectron spectrum shows that multiphoton ionization of amyl nitrite, C(5)H(11)ONO, using ultrafast laser pulses deposits up to 3.7 eV of energy into internal degrees of freedom. As a result, the molecules fragment to produce various daughter ions of masses 87, 71, 60, 57, 41, 30, 29, and 27. Absorption of an additional photon with 3 eV of energy by the ions yields transients with picosecond decay times, revealing the time scale of the decomposition dynamics of the initially prepared parent ion. Each mass peak has a distinct time constant, in the range of 1.2 to 7.9 ps, emphasizing the dependence of the fragmentation mechanism on the ion internal energy.

Time-resolved molecular frame dynamics of fixed-in-space CS2 molecules.

Random orientation of molecules within a sample leads to blurred observations of chemical reactions studied from the laboratory perspective. Methods developed for the dynamic imaging of molecular structures and processes struggle with this, as measurements are optimally made in the molecular frame. We used laser alignment to transiently fix carbon disulfide molecules in space long enough to elucidate, in the molecular reference frame, details of ultrafast electronic-vibrational dynamics during a photochemical reaction. These three-dimensional photoelectron imaging results, combined with ongoing efforts in molecular alignment and orientation, presage a wide range of insights obtainable from time-resolved studies in the molecular frame.

Excited-state dynamics of isolated DNA bases: a case study of adenine.

We present a summary of recent advances in the understanding of the UV photophysics of the isolated DNA base adenine, emphasizing a discussion of the mechanisms behind the ultrafast relaxation following excitation to the pipi* band. Drawing on our femtosecond time-resolved photoelectron spectroscopy experiments, we discuss differences in the ultrafast relaxation of adenine and 9-methyladenine and consider the relative merits of the various proposed mechanisms.

Role of rotational temperature in adiabatic molecular alignment.

One-dimensional alignment of molecules in the adiabatic limit, where the pulse duration greatly exceeds the molecular rotational periods, is studied experimentally. Four different asymmetric top molecules (iodobenzene, p-diiodobenzene, 3,4-dibromothiophene, and 4,4'-dibromobiphenyl), rotationally cooled through a high pressure supersonic pulsed valve, are aligned by a 9-ns-long pulse. Their orientations are measured through Coulomb explosion, induced by a 130-fs-long pulse, and by recording the direction of the recoiling ions. The paper focuses on the crucial role of the initial rotational temperature for the degree of alignment. In particular, we show that at molecular temperatures in the 1 K range very strong alignment is obtained already at intensities of a few times 10(11) W/cm2 for all four molecules. At the highest intensities (approximately 10(12) W/cm2) the molecules can tolerate without ionizing >or=0.92 in the case of iodobenzene. This is the strongest degree of alignment ever reported for any molecule.

Nonadiabatic alignment of asymmetric top molecules: rotational revivals.

The rotational revival structure of asymmetric top molecules, following irradiation by an intense picosecond laser pulse, is explored theoretically and experimentally. Numerically we solve nonperturbatively for the rotational dynamics of a general asymmetric top subject to a linearly polarized intense pulse, and analyze the dependence of the dynamical alignment on the field and system parameters. Experimentally we use time-resolved photofragment imaging to measure the alignment of two molecules with different asymmetry, iodobenzene, and iodopentafluorobenzene. Our numerical results explain the experimental observations and generalize them to other molecules. The rotational revival structure of asymmetric tops differs qualitatively from the intensively studied linear top case. Potentially it provides valuable structural information about molecules.

Observation of enhanced field-free molecular alignment by two laser pulses.

We show experimentally that field-free alignment of iodobenzene molecules, induced by a single, intense, linearly polarized 1.4-ps-long laser pulse, can be strongly enhanced by dividing the pulse into two optimally synchronized pulses of the same duration. For a given total energy of the two-pulse sequence the degree of alignment is maximized with an intensity ratio of 1:3 and by sending the second pulse near the time where the alignment created by the first pulse peaks.

Nonadiabatic alignment of asymmetric top molecules: field-free alignment of iodobenzene.

Nonadiabatic laser alignment of an asymmetric top molecule is studied using the combination of a quantum dynamical theory and time-resolved photofragment imaging experiments. In particular, the degree of alignment of iodobenzene, induced by an intense, linearly polarized picosecond laser pulse, is calculated and measured. Pronounced alignment is obtained under field-free conditions.