Takashi Nakajima

We show that complete population transfer is not in general possible through continuum intermediate states. We present a formal theoretical argument and supporting numerical results. In addition, the behavior of the system is compared with the well-known \ensuremath{\Lambda} system.

We show that the autoionization line shape can be modified by the choice of two laser intensities and the relative phase. Under well specified conditions, proper choice of the phase can lead to selective cancellation of the transition to the discrete or the continuum part of the state. Application of the idea to a multichannel problem in the rare gases also provides significant effects.

We theoretically investigate the effects of the carrier-envelope phase of few-cycle laser pulses in the multiphoton ionization regime. For atoms with low ionization potential, total ionization yield barely exhibits phase dependence, as expected. However, population of some bound states clearly shows phase dependence. This implies that the measurement of the carrier-envelope phase would be possible through the photoemission between bound states without energy-and-angle-resolved photoelectron dete

We present a formal theory and detailed calculations for phase-dependent laser-atom interactions involving autoionizing states. First, through simple models, we demonstrate that the simultaneous one- and three-photon excitation of one or two neighboring autoionizing states can exhibit profound changes of the line shape, as the relative phase of the two fields is varied from 0 to \ensuremath{\pi}. Through a proper choice of the field intensities and the phase, we obtain analytical results showing

We investigate a simple scheme for autocorrelation measurement of an xuv pulse. It is based on double ionization of He. We have found that, in a certain photon energy range, the detection of doubly charged positive ions instead of energy-resolved photoelectrons is sufficient for autocorrelation, which greatly simplifies the detection system for practical use.

Abstract Routinely processed paraffin sections from 20 patients with adult laryngeal papillomas were examined for the presence of human papillomavirus type 11 (HPV‐11) DNA and its specific mRNA by in situ hybridization methods using 35 S‐labeled RNA probes. Immunohistochemical techniques were also used to identify papillomavirus genus‐specific common antigen (pgsantigen). HPV‐11 DNA signals and/or papillomavirus genus‐specific common antigen were detected in all eight samples of multiple larynge

We theoretically investigate above-threshold ionization by chirped laser pulses. By comparing the photoelectron energy spectra and the photoelectron angular distributions of Na for the laser pulses with different chirp rates but with the identical spectral profile, we find that the ionization processes have a clear dependence on the chirp rate. Further calculations without excited bound states during the time propagation of the wave function reveal practically no chirp dependence, which is clear

We present a theory of modulating ionization by controlling the phases of incident laser fields. Specific calculations have been performed for the Na atom using the density-matrix equations. Significant modulation of the ion signal can be obtained by choosing the appropriate combinations of laser intensities and frequencies, in resonant as well as nonresonant processes.

We present a theory of phase control which governs the time-dependent behaviour of a system involving one bound and one decaying discrete state embedded in several continua belonging to several ionization thresholds. Representative numerical results are shown, focusing on the behaviour of the branching ratio as a function of the relative phase of two lasers, detuning, and Rabi frequencies (or intensities). Specific quantitative results on the Ca atom are also reported.

The nonlinear response of an autoionizing medium in which coherence is established essentially by nonradiative interactions is analyzed. It is found that, by proper tuning of two radiation fields, the third-order nonlinearities can be enhanced while the absorption is canceled. The proposed scheme is particularly useful for enhancing nonlinearities in the VUV region.

We theoretically study the dependence of atomic excitation and ionization on the carrier envelope phase of few-cycle laser pulses in the multiphoton ionization regime. Our theoretical results for the hydrogen atom based on the solution of the 3D time-dependent Schrödinger equation show that the strong phase dependence can be seen in not only total ionization, but also bound-state population under the weak laser intensity regime.

We study the level shifts and depopulation of Rydberg states, confined between two parallel metallic plates, due to blackbody radiation. After the derivation of the formal equations for the level shift and the depopulation, we present, as an example, specific calculations for the Rb $24s$ state as a function of plate distance $L$ at three different temperatures 10, 85, and 205 K.

We present theoretical results on the spin-polarization of photoelectrons ejected in the process of single-, two-, and three-photon ionization of xenon by circularly polarized radiation. In all cases, we find significant spin-polarization at a series of photon frequency ranges together with substantial cross-sections. The results are analysed and discussed from the perspective of spin-polarized electron sources for which they seem rather promising.

We theoretically investigate above-threshold ionization of Mg by linearly and circularly polarized fs laser pulses. We find that the above-threshold ionization peaks are accompanied by small subpeaks for both linearly and circularly polarized pulses. We interpret the physical origin of the subpeaks as above-threshold ionization from the low-lying bound states which are far off-resonantly excited by the spectral wing of the pulse. This interpretation is confirmed by our comparative numerical stud