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We examined dissociative ionization of O2 in an intense femtosecond laser field (782 nm, 120 fs, 4 × 1014 W cm−2) by recording the kinetic energy distribution of O+ emitted along the laser polarization direction as a function of the delay time between the pump pulse (9 × 1013 W cm−2) for the molecular alignment and the probe pulse for the dissociative ionization. We found the two distinct rotational revival patterns which are out-of-phase by π with each other in the kinetic energy distribution of O+. One of the patterns shows the dissociative ionization is enhanced when the O2 axis is parallel to the laser polarization direction, suggesting that the ionization is induced by the electron emission from the 3σg orbital. On the other hand, the other pattern shows that the dissociative ionization is enhanced when the O2 axis is perpendicular to the laser polarization direction, suggesting that the ionization is induced by the electron emission from the 1πu orbital. Because of the collection efficiency of the time-of-flight mass spectrometer, the enhancement of the O+ yield at the anti-alignment time delay indicates that the electron emission from the 1πu orbital is followed by the molecular alignment of O2+ in the course of the dissociation. We performed classical trajectory Monte-Carlo simulation of O2+ with the dissociation and rotational coordinates in the light-dressed potential to evaluate the effect of the post-ionization alignment by the probe pulse. [ABSTRACT FROM AUTHOR]

We examine the dependences of the single and double ionization probabilities of NO radical on the angle between the NO axis and the laser polarization direction in an intense laser field (790 nm, 100 fs, 1–10 × 1014 W/cm2) and show that the double ionization is enhanced when the NO axis is parallel to the laser polarization direction. We reveal that the angular dependence of the sequential double ionization probability is determined by the shape of the 5σ orbital of NO+ from which the second photoelectron is emitted in the ionization from NO+ to NO2+. We also reveal that the fast oscillation in the probability of the tunnel ionization of NO originating from a coherent superposition of the two spin–orbit components in the electronic ground X2Π state is described well based on the molecular Ammosov-Delone-Krainov (MO-ADK) theory in which the time evolution of the electron density distribution of the 2π orbital is taken into account. [ABSTRACT FROM AUTHOR]

We numerically investigate the rotational dynamics of NO in the electronic ground X2Π state induced by an intense two-color laser field (10 TW/cm2) as a function of pulse duration (0.3–25 ps). In the short pulse duration of less than 12 ps, rotational Raman excitation is effectively induced and results in molecular orientation. On the contrary, when the pulse duration is longer than 15 ps, the rotational excitation is suppressed. In addition to the rotational excitation, we find that transitions between Λ-type doubling are induced. Significantly, the maximum coherent wave packet between Λ-type doubling in J = 0.5 is generated using the pulse duration of 19.8 ps. The wave packet changes to the eigenstates of Λ = +1 or −1 alternatively, where Λ is the projection of the electronic orbital angular momentum on the N–O axis, which is regarded as the unidirectional rotation of an unpaired 2π electron around the N–O axis in a space-fixed frame as well as in a molecule-fixed frame. The experimental method to observe the alternation of the rotational direction of the electron around the N–O axis is proposed. [ABSTRACT FROM AUTHOR]

We investigate the carrier-envelope phase (CEP) dependences of the single and double ionization processes of methanol (CH3OH) in an intense near-IR few-cycle laser field (2.1 × 1014 W/cm2) by the asymmetry in the ejection direction of CH3+ for the non-hydrogen migration channels and CH2+ for the hydrogen migration channels created through the C–O bond breaking after the ionization. Based on the absolute CEP values at the laser–molecule interaction point, calibrated by the method using intense few-cycle circularly polarized laser pulses [Fukahori et al., Phys. Rev. A 95, 053410-1–053410-14 (2017)], we confirm that methanol cations are produced by tunnel ionization and methanol dications are produced by the recollisional double ionization. We obtain the phase offset for the double ionization accompanying no hydrogen migration to be 1.85π as the absolute CEP at which the extent of the asymmetry becomes maximum. We interpret the phase shift of 0.85π from the phase offset of 1.0π for the tunnel ionization, estimated by a tunnel ionization model incorporating the chemical bond asymmetry, as the corresponding time delay associated with the electron recollisional ionization. The positive phase shift of 0.13π for the single ionization in the non-hydrogen migration channel is interpreted as the additional time (165 as) with which a methanol cation can be excited electronically prior to the decomposition. The additional phase shift of 0.22π for the single ionization in the hydrogen migration channel is interpreted as the additional time (280 as) required for a methanol cation to be excited electronically leading to the hydrogen migration prior to the decomposition. [ABSTRACT FROM AUTHOR]

We theoretically investigate the creation of the rotational wave packet in molecular ions upon the strong-field ionization of diatomic molecules by a femtosecond laser pulse. The rotational excitation of molecular cation is ascribed to the dependence of the strong-field ionization probability on the orientation angle of the molecular axis with respect to the laser polarization direction. By extending the molecular strong-field approximation theory, we calculate the rovibrational state distribution of the ground electronic X 1 Σ + state of NO+ after the ionization of NO in the electronic ground X 2 Π 1 / 2 state. We also show that the extent of the rotational excitation of NO+ is enhanced by the stimulated impulsive Raman processes both in neutral NO and in NO+ cation. The resultant time evolution of the rotational wave packet of NO+ is in good agreement with the delay dependent NO2+ yield recorded experimentally in our previous pump–probe study. [ABSTRACT FROM AUTHOR]

An analytical formula representing the photoelectron kinetic energy at which the ejection direction of photoelectrons generated by an intense circularly-polarized few-cycle laser pulse flips was derived and was used for determining the laser pulse duration.

We observed the dynamical evolution of the electron–nuclear wavepacket composed of the and 32Πu states of with an evolution period of 1.3 fs. We created a dissociative wavepacket of by photoionization of O2 with the irradiation of an intense attosecond pulse train (APT) in the extreme ultraviolet (XUV) wavelength region, and probed it with another XUV APT by scanning delay between the two APTs. We revealed the evolution of the electron–nuclear wavepacket by the analysis of the spectral phase in the Fourier transform of the recorded delay dependent momentum spectrum of O+. We also found the correlation between the photon energy and the released kinetic energy originating from the dissociation of Rydberg states of O2 followed by the autoionization of the fragment O atom in the Rydberg states. [ABSTRACT FROM AUTHOR]

We examined dissociative ionization of O 2 in an intense femtosecond laser field (782 nm, 120 fs, 4 × 10 14 W cm -2 ) by recording the kinetic energy distribution of O + emitted along the laser polarization direction as a function of the delay time between the pump pulse (9 × 10 13 W cm -2 ) for the molecular alignment and the probe pulse for the dissociative ionization. We found the two distinct rotational revival patterns which are out-of-phase by π with each other in the kinetic energy distribution of O + . One of the patterns shows the dissociative ionization is enhanced when the O 2 axis is parallel to the laser polarization direction, suggesting that the ionization is induced by the electron emission from the 3σ g orbital. On the other hand, the other pattern shows that the dissociative ionization is enhanced when the O 2 axis is perpendicular to the laser polarization direction, suggesting that the ionization is induced by the electron emission from the 1π u orbital. Because of the collection efficiency of the time-of-flight mass spectrometer, the enhancement of the O + yield at the anti-alignment time delay indicates that the electron emission from the 1π u orbital is followed by the molecular alignment of O 2 + in the course of the dissociation. We performed classical trajectory Monte-Carlo simulation of O 2 + with the dissociation and rotational coordinates in the light-dressed potential to evaluate the effect of the post-ionization alignment by the probe pulse.