Photo-versus pulsed field-ionization of individual high n (n less than or similar to 57) Rydberg states: Resolved d,g series of benzene

Low l Rydberg states of the polyatomic molecule benzene in the range 37 les s than or similar to n less than or similar to 56 are resolved for the firs t time by double resonance excitation using narrow-band ultraviolet (UV)-la ser pulses. The short lived Rydberg states are detected by photoexcitation of the ionic core with the same laser pulses followed by fast autoionizatio n of the Rydberg states. The Rydberg spectrum obtained shows clear substruc ture of individual high n Rydberg peaks with components of quantum defects delta(l)=0.00, 0.015, and 0.04. It is compared with a delayed pulsed field ionization spectrum with a delay of 8 mu s. The bright low l core-penetrati ng zero order states cannot be observed in the delayed pulsed field ionizat ion experiment for n < 50 since there is no mixing with the long-lived Star k manifold of noncore-penetrating high l Rydberg states in the residual ele ctric stray fields present in our apparatus which is necessary to lengthen the lifetime to survive the delay of 8 mu s. Our results for the observed l ow l Rydberg states agree with previous results from Rydberg spectroscopy o f low n Rydberg states in benzene. This is an important step towards observ ation of continuous Rydberg series from n=3 up to n=56. Above n=56 the diff erent n,l Rydberg states are mixed in the electric stray fields into a sing le n Stark manifold. (C) 2000 American Institute of Physics. [S0021-9606(00 )01202-2].