Oleg L. Polyansky et al- The Spectrum of Hot Water: Rotational Transitions and Difference Bands in...
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JOURNAL OF MOLECULAR SPECTROS COPY 186, 213–221 (1997) ARTICLE NO. MS977443 The Spectrum of Hot Water: Rotational Transitions and Difference Bands in the (020), (100), and (001) Vibrational States Oleg L. Polyansky,* ,1 Jonathan Tennyson,* and Peter F. Bernath† ,2 * Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, United Kingdom; and † Departments of Chemistry and Physics, University of Waterloo, Waterloo, Ontario, Canada N2L 3G1 Received March 24, 1997; in revised form July 17, 1997 Analysis of the hot H 2 16 O spectrum, presented by Polyansky et al. (1996, J. Mol. Spectrosc. 176, 305–315), is extended to higher vibrational states. Three hundred thirty mainly strong lines are assigned to pure rotational transitions in the ( 100) , (001 ), and ( 020) vib rat iona l sta tes . The se lines, which invol ve sig nifi can tly higher rota tion al energy levels than were known previously, are assigned using high-accuracy variational calculations. Transitions in (020) are assigned up to K a Å 18, compared with the maximum K a of 10 known previously. Crossings of vibration–rotation energy levels result in the observation of extra intensity-stealing transitions. In particular, this leads to the assignment of ( 020) – ( 100) and (100 ) – ( 020) rotati ona l differ enc e band tran siti ons in additi on to the con ven tion al pure rota tion al lines in (020) and (100) states. These extra lines increase the number of transitions and they are likely to complicate the pure rotational water spectrum in higher excited vibrational states to an even greater extent. A few lines from our previous work on the pure rotational spectrum of hot water in the (000) and (010) vibrational states are also reassigned and some further assignments are made. 1997 Academic Press I. INTRODU CTION calculation using a highly accurate potential energy surface ( 5– 8 ). Thus, for example, Polyansky et al. ( 5 ) performed a variational fit to energy levels with J less than 15, but The spec trum of hot water has a number of impor tant appl i- obtained excellent agreement with the highest observed lev- cat ion s bu t pre sents a maj or ex per ime nta l and the or eti ca l ch al- els up to J Å 35. When we used variational calculations in lenge. For example, detailed high-resolution spectra of sun- the analy sis of the n 2 ma nif old, we were able to gr ea tly spots covering a large range in the infrared have been ob- extend the range of both J and K a levels. In addition we se rv ed ( 1, 2 ). The se de ns e sp ectra ar e al mo st totall y detected transitions in the previously unknown 5n 2 0 4n 2 unass ign ed. In ord er to ass ign thi s sp ect rum and other ho t band ( 9 ). water spectra we have been analyzing the laboratory spectra The motiv ation for the pre se nt work wa s to as sig n the of hot H 2 16 O. remaining strong transitions in the water spectrum presented In a recent paper, henceforth known as I, Polyansky et al. in I. These assignments are needed to provide a basis for ( 3 ) presented measurements of the 1550C water spectrum in the ana lysi s of the many weaker line s also pres ent. An und er- the region 400–900 cm 01 . Mo re th an 40 00 lin es we re ob - standing of both strong and weak lines is necessary to under- served and about 600 were assigned as lines belonging to pure stand the very dense hot water spectrum in sunspots ( 1, 2 ) . rotational transitions in the (000) and (010) vibrational states. As will be shown, thes e assi gnments sign ific antl y extend At least 300 very strong lines remained unassigned. our knowledge of the excited energy levels of water in the The assignments in I were made using a Pade ´ –Borel ef- ground and excited vibrational states. fective Hamiltonian ( 4 ) and a procedure suitable only for The extension of the water energy levels to high values of working with isolated vibrational states. The accuracy of the K a is well known to be particularly difficult. Levels with high prediction of the highest rotational energy levels was about K a are very se nsi tiv e to the sma ll ang le reg ion s of the po te nti al 2 cm 01 , which was not sufficient for a complete analysis of energy surface which are relatively poorly characterized by a spectrum with a line density of about 15 lines per cm 01 . the available experimental data ( 5, 6 ). The high K a levels are One reason for this low accuracy was the relatively small also strongly affected by adiabatic effects due to the break- number of constants used to fit the data. down of the Born–Oppenheimer approximation ( 7 ). As pointed out in I, a natural method for analyzing the In this work, we extend K a assignments for (010) from data which involves tens of vibrational states is a variational 19 (given in I) to 21; for (001) the highest derived K a is now 20, compared to 16 given by Flaud et al. ( 10 ); for the 1 Permanent address: Institute of Applied Physics, Russian Academy of ground and (100) vibrational states we increase K a to 24 Science, Uljanov Street 46, Nizhnii Novgorod, Russia 603024. from 23 (in I) and to 20 from 19 ( 10 ), respectively. A major 2 Also: Depart ment of Chemi stry, Universi ty of Arizo na, Tucson, AZ 85721. advance has been made for the (020) state where the highest 213 0022- 2852/97 $25.00 Copyright 1997 by Academic Press All rights of reproduction in any form reserved.
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