h e r The Cavendish L a b o r a t o r y , Madingley Road, Cambridge CB3 OHE, U.K. A p rim ary difficulty in o b tain in g radiofrequency sp ectra is th a t th e size of th e q u an tu m is usually m uch sm aller th a n kT, so th a t u pper and lower states of a tra n sitio n have p ractically equal po p u latio n s an d an y resulting abso rp tio n m u st be w e a k ; abso rp tio n an d stim u lated em ission rates will be very sim ilar. This difficulty is circum vented by using a laser to

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... ulate one of th e states, while a fu rth e r gain is o btained by detectin g a laser q u an tu m ( r |> k T /h ) following ab sorption of a radiofrequency q u an tu m ( v^ k T /h ). A carbon dioxide laser v ib ra tio n -ro ta tio n tran sitio n s in th e 10 pm region and, by using an expanded laser beam of d iam eter 35 m m an d an interactio n length of some 10 m, radiofrequency sp ectra are o b tain ed a t linew idths below 20 kHz. This approaches th e lim it im plied by th e tra n s it tim e for a molecule trav ersin g th e laser beam , and co n trasts strongly w ith earlier w ork using a radiofrequency cell w ithin th e laser cavity. A rate-eq u atio n model of th e experim ent is explored. The new resolution and precision available are applied to hyperfine tran sitio n s in th e ground and r 6 = 1 states of CH 3I. I t is shown th a t, although cu rren t ham iltonians represent m ost of the hyperfine stru ctu re well, a new term to represent v ib ratio n -n u clear m agnetic coupling m ust be introduced. Finally, a new in terp reta tio n is p u t on th e hyperfine spectrum of C H 3C1. Proc. R. Soc. Loud. A (1994) 18-2 cavity length during survey scans to record th e m axim um num ber of double resonance transitions accessible, for stabilizing the laser on a p articu lar absorption, and for signal averaging by m ultiple scanning. Spectra were tran sferred directly to an IBM m ainfram e for processing as required. Figure 2 shows a survey scan of hyperfine transitions in m ethyl chloride, exploiting the well-known resonance between the 9P(26) laser line and r 6 *Q3(6) tran sitio n . The c o n tra st betw een intracavity and extra-cav ity operation is quite striking. 546 A. J. Gray and R. J . Butche