Investigations of the role of ponderomotive forces in ATI using subpicosecond laser pulses.

The role of the ponderomotive force in Above Threshold Ionization (ATI) is explored using variable length pulses of 616 nm light ranging from 15 to 0.4 psec. These pulsewidths span the long- pulse to short pulse regime for ATI. Experimental results show that the ATI peaks broaden and shift with decreasing pulsewidth, and that for the shortest pulsewidth, the individual ATI peaks breakup into a series of fine structure peaks with instrument- limited widths. We analyze our data using a time-dependent ponderomotive potential and show that ponderomotive forces can account in detail for the observed shifts and broadening.