Ionization efficiency of Doppler-broadened atoms by transform-limited and broadband nanosecond pulses: one-photon resonant two-photon ionization of muoniums

We theoretically study the ionization efficiency of Doppler-broadened atoms through a one-photon resonant two-photon ionization process using nanosecond pump and ionizing laser pulses. Under the presence of significant (> tens of GHz) Doppler broadening, the bandwidth of the transform-limited nanosecond pump pulse is far smaller than the Doppler width, and the use of the broadband nanosecond pulse rather than the transform-limited nanosecond pulse seems to be much more efficient to pump and eventually ionize atoms. It turns out, however, that the transform-limited nanosecond pump pulse with sufficient high intensity can outperform the broadband nanosecond pump pulse in the high intensity regime for the ionizing pulse, at which the other broadening mechanisms because of the strong pump and fast ionization play more important roles than the bandwidth of the pump pulse. Using a set of density matrix equations, we present specific numerical results for muoniums ( mu super(+)e super(-)) with enormous Doppler widths (80 and 230 GHz), which support the above argument.