LINESHAPE AND SENSITIVITY OF SPECTROSCOPIC SIGNALS OF N$_2^+$ IN A POSITIVE COLUMN COLLECTED USING NOISE IMMUNE CAVITY ENHANCED OPTICAL HETERODYNE VELOCITY MODULATION SPECTROSCOPY

Challenges to studying gas phase ions include the dilute analyte, Doppler line broadening, and a lack of ion/neutral discrimination. Techniques which provide high sensitivity, sub-Doppler features, and some form of ion/neutral discrimination increase the ability to study gas phase ions. Recently our group has used noise immune cavity enhanced optical heterodyne velocity modulated spectroscopy (NICE-OHVMS) to help overcome each of these challenges. \hspace{0.25in}Using NICE-OHMS to probe a velocity modulated positive column produces a distinctive line shape. The high optical power from and geometry of the cavity saturates optical transitions and allows sub-Doppler Lamb dips to be observed. Depending on sideband frequency (1 or 9 times the free spectral range) the sub-Doppler features are closer together or further apart. The sub-Doppler features can then be used to measure the line-centers with high ($\sim $1 M H z) p r e c i s i o n a n d a c c u r a c y u s i n g a n o p t i c a l f r e q u e n c y c o m b . T h e K r a m e r s − K r o n i g r e l a t i o n s d e s c r i b e h o w t h e a b s o r p t i o n a n d d i s p e r s i o n a r e r e l a t e d t o o n e a n o t h e r a n d c a n b e u s e d t o o b t a i n t h e a b s o r p t i o n f r o m t h e d i s p e r s i o n (a n d v i c e − v e r s a). O w i n g t o t h e p h a s e d e p e n d e n t a b s o r p t i o n s i g n a l p r o d u c e d w i t h h e t e r o d y n e s p e c t r o s c o p y, b o t h a b s o r p t i o n a n d d i s p e r s i o n s i g n a l s c a n b e o b t a i n e d s i m u l t a n e o u s l y . T w o R F m i x e r s (o n e f o r a b s o r p t i o n a n d o n e f o r d i s p e r s i o n), e a c h d r i v i n g i t s o w n l o c k − i n a m p l i f i e r, a r e u s e d t o o b t a i n a s i g n a l f o r i o n s a n d e x c i t e d n e u t r a l s . W e w i l l r e p o r t a c o m p a r i s o n o f t h e s e n s i t i v i t i e s o f s e v e r a l a b s o r b a n c e t e c h n i q u e s t o s t u d y a n i t r o g e n i c v e l o c i t y m o d u l a t e d p o s i t i v e c o l u m n i n c l u d i n g : d i r e c t a b s o r p t i o n, c a v i t y e n h a n c e d v e l o c i t y m o d u l a t i o n, h e t e r o d y n e s p e c t r o s c o p y a n d N I C E − O H M S, a n d s h o w h o w t h e s i g n a l − t o − n o i s e r a t i o i s i n c r e a s e d b y u s i n g N I C E − O H M S . F u t u r e p l a n s f o r t h i s t e c h n i q u e i n c l u d e u s i n g a h i g h p o w e r c w − O P O i n t h e m i d − I R t o p e r f o r m h i g h p r e c i s i o n v i b r a t i o n a l s p e c t r o s c o p y o f i o n s s u c h a s C H$ _5^+$.

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Author Institution: Department of Chemistry, University of Illinois at Urbana-Champaign, Urbana, IL 61801; Departments of Chemistry and Astronomy, University of Illinois at Urbana-Champaign, Urbana, IL 61801

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http://hdl.handle.net/1811/49248

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Abstracts of OSU International Symposium on Molecular Spectroscopy 2010-2013

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