Multiphoton up-down-up-...(UDU) excitation in a two-level system was r
ealized under adiabatic conditions. Spectra of C2H4 excitation were in
vestigated and UDU peaks were observed. The dependence of peak amplitu
des on the powers and the frequencies of exciting fields were investig
ated experimentally as well as by computer simulation based on the rot
ating wave approximation (RWA). Agreement between experimental and the
oretical results was observed. (C) 1996 American Institute of Physics.