  High Harmonic Generation (HHG) creates coherent high frequency radiation via
the process of strong field ionization followed by recombination. Recently, a
complementary approach based on Frustrated Tunnel Ionization (FTI) was
demonstrated (Nature Photonics 12, 620 (2018)). It uses spectrally separated
peaks created by lower quantum number Rydberg states to produce coherent
extreme ultraviolet (EUV) light. While much is understood about enhancing
emission from HHG by controlling recombining electron trajectories, relatively
little is known about controlling the quantum number distribution of Rydberg
states. This distribution is generally believed to be determined primarily by
field strength and laser frequency. We show that, in fact, it also changes
significantly with the duration of the laser pulse: increasing pulse duration
depletes lower-lying Rydberg states, thereby substantially decreasing EUV
yield. Using electron trajectory analysis, we identify elastic recollision as
the underlying cause. Our results open a door to greater control over
production of coherent high frequency radiation, by combining FTI and HHG
mechanisms, and also improved the interpretation of molecular imaging
experiments that rely on elastic electron recollision.
