Gradual broadening of successive above-threshold-ionization peaks

We present theoretical calculations that show a systematic increase of the spectral widths of the successive above-threshold-ionization (ATI) peaks using Fourier-limited pulses. The increase of the width is attributed to the effective Fourier width corresponding to the higher-order processes. Based on that interpretation, we propose a simple model of calculating the width of a Nth-order ATI peak, We compare the results obtained from this model to the results obtained through the direct propagation of the time-dependent Schrodinger equation. For this, we have selected the negative hydrogen ion that represents the simplest possible system to study this feature due to the absence of resonances, that may obscure the results. Considering that Fourier-limited laser pulses with duration of the order of few tens of fs and wavelengths around the optical regime are nowadays readily available, we address the question about the nature of the photoelectron energy spectrum when the atoms are exposed in such fields.