Applicability of Resonant Two Photon Ionization in Supersonic Beams to Halogenated Aromatic Hydrocarbons

T h i s work investigates problems encountered in the application of one color resonant two-photon ionization as an ionization source in supersonic beam mass spectroscopy. Of particular interest is the fact that photons of one color may not provide sufficient energy to cause ionization even when the laser source is tuned to an excited vibronic molecular state. We have therefore correlated trends in ionization potential with molecular structure for simple systems, specifically, halogenated aniline, phenol and toluene derivatives and monoand di-substituted benzenes. In the case of para-substituted compounds where there i s l i t t l e s u b s t i t u e n t g r o u p i n t e r a c t i o n i o n i z a t i o n o c c u r s e f f i c i e n t l y a t t h e S ~ S. o r i g i n p r o v i d e d u l t r a f a s t p r o c e s s e s a r e n o t a c t i v e a s i n ~ h e 1 c a s e o f i o d o s u b s t i t u t e d b e n z e n e s . Many o r t h o c o m p o u n d s , h o w e v e r , a r e f o u n d n o t t o i o n i z e e f f i c i e n t l y . T h i s i s p r o b a b l y due t o a c o m b i n a t i o n o f c o u l o m b i c a n d s t e r i c i n t e r a c t i o n s w h i c h r e s u l t i n a n i n c r e a s e i n i o n i z a t i o n p o t e n t i a l . T h e s e t y p e s o f e f f e c t s h a v e b e e n q u a l i t a t i v e l y r e l a t e d t o t h e e l e c t r o n r e l e a s i n g and w i t h d r a w i n g p r o p e r t i e s o f t h e s u b s t i t u e n t g r o u p s t h e r e b y a l l o w i n g r e a s o n a b l e p r e d i c t i o n s t o be made r e g a r d i n g t h o s e t y p e s o f s u b s t i t u t e d b e n z e n e s w h i c h c a n be p r o b e d with RZPI.