Nonresonant Multiphoton Ionization of Stark Decelerated Molecules by Femtosecond Laser Pulses

Nonresonant multiphoton ionization by femtosecond laser pulses can be applied to any molecule virtually,thereby greatly enhancing the scope of Stark decelerated molecules.For comparison,we detect decelerated and trapped ammonia molecules using two different schemes:(ⅰ) nonresonant multiphoton ionization using intense femtosecond(fs) pulses in the near infrared,and(ⅱ) resonance-enhanced multiphoton ionization using nanosecond(ns) pulses from a tunable UV laser.The observed number of ions per shot for both schemes is similar.The fs laser detection scheme suffers from an increased background,which can be effectively eliminated by subsequent mass and velocity selection.To determine the detection volume of the ns laser detection scheme,we present measurements in which the decelerated ammonia molecules are bunched to a packet with a longitudinal spread well below~100 μm.It is concluded that the detection volume for the ns laser detection scheme is 1.5-2 times larger than that of the fs laser detection scheme.

Field intensity dependence of the dissociative multiple ionization of argon dimers in strong femtosecond laser fields

The dissociative ionization of Ar dimers is investigated in femtosecond laser fields with intensities from 260 to 1020 TW/cm^(2).The three-dimensional momentum and kinetic-energy release of fragmental ions generated from the channels Ar_(2)^(2+)→Ar^(+)+Ar^(+),Ar_(2)^(3+)→Ar^(2+)+Ar^(+),and Ar_(2)^(4+)→Ar^(2+)+Ar^(2+)were measured with a cold-target recoil-ion momentum spectrometer.It is shown that the laser intensity significantly modulates the kinetic energies and angular distributions of fragmental ions from dissociative double ionization.Laser-induced charge-transfer following one-site double ionization contributes relatively more to the dissociative double ionization at lower laser intensity.The calculation results of a one-dimensional model based on the WKB approximation suggest that the charge transfer is suppressed at higher laser intensity due to the core polarization effect.In addition,double,triple,and quadruple dissociative ionizations of Ar dimers are accompanied by frustrated-tunneling ionization that increases with the laser intensity.

Electron localization enhanced photon absorption for the missing opacity in solar interior

The internal solar structure predicted by the standard solar model disagrees with the helioseismic observations even by utilizing the most updated physical inputs, such as the opacity and element abundances. By increasing the Rosseland mean, the decade-old open problem of the missing opacity can be resolved. Herein, we propose that the continuum electrons in the radiative processes lose phases and coherence as matter waves, giving rise to a phenomenon of transient spatial localization. It not only enhances the continuum opacity but also increases the line widths of the bound-bound transitions. We demonstrate our theoretical formulation by investigating the opacity of solar mixtures in the interior. The Rosseland mean demonstrates an increase of 10%-26% in the range of 0.3 R⊙-0.75 R⊙. The results are compared with the recent experimental data and the existing theoretical models. Our findings provide novel clues to the open problem of the missing opacity in the solar interior and new insight on the radiative opacity in the hot dense-plasma regime.

Phase dependence of third-order harmonic generation in gases induced by two-color laser field

We experimentally demonstrate third-harmonic generation(THG)in gases ionized by a femtosecond laser pulse superimposed on its second-harmonic(SH).The mechanism of THG has been investigated,and it demonstrates that a third-order nonlinear process dominates at low pump intensity.Asymmetric third-harmonic(TH)spectra are observed at different time delays in two color fields,which are attributed to the process of the four-wave mixing(FWM)of the broad spectrum of pump pulses.A joint measurement on the terahertz(THz)and the TH is performed.It reveals that the optimized phase for the THG jumps from 0 to 0.5πas the pump intensity increases,which is different from the THz being a constant,and indicates that the THG arises from the nonlinearity of the third-order bound electrons to the tunnel-ionization current.