Slow Electrons Generated by Intense High-Frequency Laser Pulses

A very slow electron is shown to emerge when an intense high-frequency laser pulse is applied to a hydrogen negative ion. This counterintuitive effect cannot be accounted for by multiphoton or tunneling ionization mechanisms. We explore the effect and show that in the high-frequency regime the atomic electron is promoted to the continuum via a nonadiabatic transition caused by slow deformation of the dressed potential that follows a variation of the envelope of the laser pulse. This is a general mechanism, and a slow electron peak should always appear in the photoelectron spectrum when an atom is irradiated by a high-frequency pulse of finite length.

Figure 1

Typical partial-wave photoelectron spectra for ${\mathrm{H}}^{-}$ produced by high-frequency pulses with linear (LP) and circular (CP) polarizations. The laser parameters are $F=0.3$ ($I=3.1\times {10}^{15}\mathrm{W}/{\mathrm{cm}}^2$), $\omega =\pi /10=8.55\mathrm{eV}$, and $T=2400=57.6\mathrm{fs}$, so the pulse contains 120 optical cycles and ${\alpha }_0=3.04$. In both cases, the total ionization probability is close to 99%. The arrows indicate the $n$-photon absorption energies $E_0+n\omega$ for $n=1$ and 2.

Figure 2

Broken curves: the full TDSE results, Eq. (1), for the slow electron peak produced by pulses with $F=n^2{F}_0$, $\omega =n{\omega }_0$, and $T=2400$, where ${\omega }_0=\pi /10$, $F_0=0.3$, and the integer $n$ varies from 1 to 16. The value of ${\alpha }_0=3.04$ for these pulses is kept fixed and equal to that in Fig. 1. Solid curves: the results obtained by solving the time-averaged TDSE (4). The total spectra for linear (LP) and circular (CP) polarizations are shown.

Figure 3

An example of the trajectory traced by the solution $t(k)$ to Eq. (5) in the complex $t$ plane. The SS is promoted to the continuum at the critical point $t_c=t(0)$.

Figure 4

The partial-wave components of the slow electron peak produced by pulses with ${\alpha }_0=3.04$, as in Figs. 1, 2. Solid curves: the exact results obtained from the time-averaged TDSE (4). Dashed curves: the adiabatic approximation, Eq. (6). Dash-dotted curves: the limiting form of the adiabatic approximation for $T\to \infty$, Eq. (8).