J. Phys. Chem. 1986, 90, 1086-1095
1086
LASER CHEMISTRY, MOLECULAR DYNAMICS, AND ENERGY TRANSFER Probing Excited States of NO Involved in Multlstate Interactions Using the Optical-Optical Double Resonance-Multiphoton Ionization Technique Wan Yee Cheung, William A. Chupka, Steven D. Colson,* Sterling Chemistry Laboratory, Yale University, New Haven, Connecticut 0651 I
Dolores Gauyacq, Laboratoire de Photophysique Moleculaire C.N.R.S.,? Batiment 21 3, Universite de Paris-Sud, 91405-0rsay, France
Phaedon Avouris, and James J. Wynne IBM Thomas J . Watson Research Center, Yorktown Heights, New York 10598 (Received: April 22, 1985)
Many new transitions from the (3sa) A2Z+ state to higher Rydberg and valence states in I4Nl6Ohave been observed by the technique of optical-optical double resonance-multiphoton ionization. Upper states include members from the np, nd, nf Rydberg states (n = 3 to 6) and several vibrational levels of the B’II and L211 valence states. The spectral simplification aspect of the double resonance technique allows for the observation of these spectra without appreciableband overlap. Analysis of our data leads to the identificationof several previously unobserved mixed Rydberg-valence vibrational levels. Discussions of the Rydberg-valence interactions in the 68600-71 200-cm-’ energy region of l4NI6Oare presented in light of these results.
Introduction The fine structure analyses of the high-resolution absorption and emission spectra of N O have revealed a system of excited states which have been interpreted in far greater detail than many other molecu1es.I During the past few years, multiphoton ionization spectroscopy has proved to be very useful for the study of highly excited states of molecules, especially Rydberg states. This recent experimental approach is well illustrated by the extensive work on Nevertheless, most of the knowledge about the excited states of this molecule still results from highquality absorption data, as shown for instance by discussion on the multiphoton data given by Dressler and Miescher.* Recently, by combining the optical-optical double resonance technique with multiphoton ionization (OODR-MPI), valuable new information has been obtained in several energy regions corresponding to high density of states and consequently to very dense spectral regions in the absorption spectrum. For instance, OODR-MPI has provided complementary data on vibrationally excited levels of the valence state B2114,5and on low-n and high-n Rydberg A region of particular interest lies just below the dissociation limit N(2D) + O(3P)at 71 625 cm-’ to which three ’rI valence states converge. Indeed this region shows multistate interaction involving those states of *IIsymmetry, especially the npx Rydberg states and the B and L valence states. On the basis of the absorption spectrum analyses, Gallusser and dressler9 recently proposed a quantitative description of this multistate interaction by limiting this interaction to five electronic states, Le., the lowest three members of the n p r series and the B2!J and L2H valence states. With a small number of coupling parameters, they could reasonably interpret the energies, rotational constants, and oscillator strengths of the observed levels of these five electronic states in three different isotopes and predict positions and molecular parameters of yet unobserved levels. In particular, they have
’Laboratoire associe a L’Universite de Paris-Sud.
confirmed assignments of the observed vibrational progression of the poorly defined L21i state. The present work provides OODR-MPI data in the same energy region (between 68 600 and 7 1 200 cm-’ from the ground state) by pumping the A22, v = 1 level as the intermediate state. Previously unobserved (or unassigned) components of 2rI symmetry involved in a npr-B211-L211 interaction as well as several components of the np, nd, nf Rydberg states have been observed by this method. These data will be presented in comparison with the corresponding absorption data and will be discussed in the context of the theoretical predictions of Gallusser and Dressler.
Experimental Section Details of the experimental setup have been given elsewhere.6 A specific rotational level of the A22+(o=1) state is populated by a pulsed nitrogen-pumped dye laser (the “pump”) set at an appropriate rotational line of the A22+-X211 (1,O) two-photon spectrum, the setting being determined by detecting ions formed by multiphoton ionization. With this laser fixed in frequency, (1) E. Miescher and K. P. Huber, Inf. Rec. Sci., Phys. Chem. Ser. 2, 3, (1976), references therein. (2) P. M. Johnson, M. R. Berman, and D.Zakheim, J. Chem. Phys., 62, 2500 (1975). (3) S. Wallace and K. K. Innes, J . Chem. Phys., 72, 4805 (1980). (4) (a) T. Ebata, H. Abe, N. Mikami, and M. Ito, Chem. Phys. Lett., 86, 445 (1982);(b) T. Ebata, T. Imajo, N. Mikami, and M. Ito, Chem. Phys. Lett., 89, 45 (1982)fc) T. Ebata, N. Mikami, and M. Ito, J . Chem. Phys., 78, 1132 (1983). (5) Y. Achiba, K. Sato, K. Shobatake, and K. Kimura, J . Chem. Phys., 78, 5474 (1983); Bookof Abstracts XIIIth I.C.P.E.A.C., Berlin, 1983, p 68. (6) W. Y. Cheung, W. A. Chupka, S. D. Colson, D. Gauyacq, Ph. Avouris, and J. J. Wynne, J . Chem. Phys., 78, 3625 (1983). (7) M. Seaver, W. A. Chupka, S. D.Colson, and D.Gauyacq, J . Phys. Chem., 87, 2226 (1983). (8) K. Dressler and E. Miescher, J . Phys. Chem.. 75, 43 10 ( 1 98 I ) . (9) R . Gallusser and K. Dressler, J . Chem. Phys., 76, 431 1 (1982).
0022-365418612090-1086$01.5010 0 1986 American Chemical Society
Excited States of NO
The Journal of Physical Chemistry, Vol. 90, No. 6, 1986
VALENCE -
RYDBERG A
/
\
L211 - PO pn du,n d6 f ‘ -v v n v n v n v n v n v n v
cm-’f B’TT
74721.5
’ 5
1.P ( v - 0 )
i
: -6
w
0
4--il
- --31 --
t
69000 =29
Figure 1. Energy diagram of the observed excited levels in the present OODR-MPI experiments; the levels with dashed lines have not been observed in this work (see text).
a second dye laser, the “probe”, is scanned through single-photon resonances between the A2Z+(v=l) and higher Rydberg or valence levels to generate an enhanced ionization spectrum. By repeating this procedure for a number of pumped rotational levels in the A state (pump rotational lines are the same as those used in our previous work6), one obtains the entire rotational structure for each electronic transition from the A state, in the range of the probe laser wavelength scan. The PI, and PZ2banches observed for N’ = 3 to 7 were induced by an external magnetic field. Absolute laser calibration is obtained by monitoring the MPI spectra of Sr and Calo together with Fabry-Perot fringes. Repeated measurements gave line positions with an uncertainty of 2k0.2 cm-I. Signal averaging and data acquisition are done by using a gated detection scheme and an IBM 370/168 computer.
Results and Discussion General Remarks on the Observed Spectra. Figure 1 shows the different observed levels probed from the u = 1 level of the A state. Also shown are other levels that were not observed in our experiment. Unlike the lower energy region of the 5s and 4f ( u = 1) Rydberg levels, where only one electronic state was observed at a time in the OODR-MPI spectra,6 the present spectra display several electronic states in the same region as shown for example in Figures 4, 8, and 10. This is a direct consequence of the multistate Rydberg-valence interaction: transitions from the A state to valence states which are expected to be weak in the absence of any perturbation can borrow intensity from Rydberg-Rydberg transitions and can then be readily observed for a large range of rotational quantum numbers as will be seen below. For the same reason several b # 0 Rydberg-Rydberg transitions have been observed besides the strongest Ao = 0 transitions as is clearly seen in Figure 1. This last point also differs from the observations of ref 6 where only h = 0 transitions were observed. In the analysis of OODR-MPI spectra, one must be aware that the observed line strengths are much more difficult to interpret than in the case of absorption spectra. The intensities in all allowed electronic absorption spectrum are determined by four factors: the electronic transition moment, the rotational (Honl-London) ~~
~~
(10) J. A. Armstrong, P. Escherick, and J, J . Wynne, Phys. Rev. A, 15, 180 (1977); J. A. Armstrong, J. J. Wynne, and P. Escherick J. Opt. Soc. Am., 69, 211 (1979); P. Escherick, Phys. Rev. A , 15, 1920 (1977).
1087
factors, the vibrational overlap factors, and individual line shape factors. The OODR-MPI signal depends upon resonant, two-step ionization from an optically pumped level to an intermediate level and then to a autoionizing level or directly to the ionization continuum. The first absorption step gives rise to a time-dependent “initial state” preparation which is affected by all of the factors memtioned above as well as by the lifetime of the pumped level, the A(u=l) level in this work. This latter lifetime factor affects the entire spectrum of a given pump level uniformly. It can, however, give rise to different ion yields from different pump levels with different lifetimes and has been used to study rotationally enhanced predissociation in NH3,I1H20,12and H2S.I3 In the MPI process, one must consider the ionization step in addition to the absorption strength for the transition from the pumped level to the intermediate level. If the ionization cross sections were the same for all intermediate levels, then the ion yield excitation spectrum in the probe laser would faithfully represent the absorption spectrum of the pumped level. Unfortunately, this will generally not be the case for a variety of reasons. (a) If the intermediate level is short-lived, it may decay before it can be driven to ionization. (b) Certain intermediate states may also have low ionization yields because of electronic “forbiddeness”. In the case of NO, the valence excited (intermediate) states have electron core configurations leading to a one-electron excitation of the ion. These ion states cannot be reached by absorption of one photon. Thus the ionization yield of these valence states will be very low in the absence of strong configuration interactions. (c) The ionization of the intermediate level may also be Franck-Condon forbidden. Such levels can show large (random) enchancements when the ionization step is accidentally resonant with an autoionizing level which has a large Franck-Condon factor for absorption from the intermediate level. There are also factors which result from the use of focused pulsed lasers in these OODR-MPI experiments. The pump laser itself gives a background MPI signal. This is most apparent for very short-lived intermediate states which can show up as “dips” in the background ~ i g n a 1 . I ~Similarly, since the population of the pumped level is quite small (one works at low pump laser powers to minimize the background signal) absorptions to allowed intermediate states must be taken at low probe laser powers to avoid saturation while weaker absorptions can be enhanced by increasing the probe laser power. Also, the polarization of the lasers can affect the rotational line intensities and the MPI signal depends upon the spatial overlap of the two lasers. In summary, even under the best of circumstances, relative OODR-MPI line strengths are only of qualitative merit and must be interpreted with consideration for many more factors than in the case of absorption spectra. In these experiments, the lasers are overlapped in time and space by using both lasers to drive the ionization step as nearly to saturation as possible in order to minimize resonance effects in the ionization step. Furthermore, relative intensities are only compared over small regions of the spectrum. The assignments of the observed OODR-MPI lines will now be discussed. The division of the explored region into five sections is more or less arbitrary. They correspond primarily to separate experimental scans of regions containing the predominant interacting partners. The transition energies and intensities will be discussed by taking into account the results from absorption (and emission) data and the multistate interaction model proposed by Gallusser and Dressier.' 1. The L211(8)~Q211(0)-AZ+(1) Transition. A total of seven vibrational levels ( u ) = 1 to 7 ) of the inverted L2nvalence state have been observed in previous vacuum-UV absorption experi(1 1) J. H. Glownia, S. J. Riley, S. D. Colson, and G.C. Nieman, J . Chem. Phys., 72, 5992 (1980). (12) M. N. R. Ashfold, J. M. Bayley, . . and R. N. Dixon, Chem. Phys., 84, 35 (1984). (13) M. N. R. Ashfold, J. M. Bayley, and R. N. Dixon, Faraday Discuss. Chem. Soc., 75, 411 (1983). (14) W. Y . Cheung and S.D. Colson in “Advances in Laser Spectroscopy”, Vol. 2, B. A. Garetz and J. R. Lombardi, Ed., Wiley, New York, 1983.
1088 The Journal of Physical Chemistry, Vol. 90, No. 6 , 1986
Cheung et al.
6900C
6890C
Figure 2. Representative spectrum of the L(8)-A(1) transition from the N = 5 pump level
ments. Discrete features near the L‘ = 8 level have been observed but not positively identified.* In the energy range from 21 950 to 22 150 cm-I the OODRMPI spectra display a typical 2n(case(a))-’Zf transition with an upper state effective spin-orbit splitting of e 7 0 cm-I as shown for instance in Figure 2 . The upper state has been assigned to the previously unobserved u = 8 level of the L state. In the range of the observed J levels ( J = 3 / 2 to 12’/*) the lower component L2II,;, does not show any local perturbations and an effective rotational constant B$f2 = 1.252 f 0.007 cm-I can be derived from combination differences. The upper component L2111j2is obviously perturbed throughout the range of observed rotational levels and shows a small avoided crossing a t J = 6’/2 (see Figure 3). The term values of this level are plotted vs. J ( J + 1) in Figure 3. The solid lines in Figure 3 correspond to MPI data while dashed lines correspond to UV absorption data on the Q(0) state.I5 This figure illustrates the use of OODR-MPI to complement absorption data. since the components observed by OODR-MPI could not be observed in absorption and vice versa (excpet for the lowest three rotational levels of the Q2n(0), F, component for which very weak MPI lines have been detected). The upper component observed in our spectra is strongly mixed with the Q(O),F, component and corresponds to the Ll,2(8) Q(O),F, perturbed level following the labeling nomenclature of ref 16 for perturbed levels. The other component Q(O),F, LI/2(8) has only been observed in absorptioni5as mentioned above and the last component, i.e. the Q(0),F2 level, has not yet been observed. In order to understand these OODR-MPI observations, we have performed a local deperturbation involving only the two ’II levels Q(0) and L(8). The effect of the other interacting levels has been partly taken into account in form of “effective unperturbed” parameters: interaction with the B(28) and B(29) 211valence levels has been partially included into an effective spin-orbit constant of the unperturbed Q(0) level in our simplified treatment (Aa = + I 6 cm-I). In addition, I uncoupling originating from the upper lying level 5pu,R(O) affects the rotational constant and the A-type doubling parameter of the Q(0) level. We have found for the latter parameter q = A v f , / N ( N + 7) = 0.07 cm-I which leads, in the case of pure precession, to [ [ ( I I)]‘/* = 1.3 instead of 1 for the 5p complex. The significance of such values being larger than 1 for a p electron has been discussed in detail by Jungen and Miescher’’ who have suggested among the possible origins, configuration interaction or deviation from pure precession. The “unperturbed rotational constant” of this level in our two-level interaction calculation is BQ = 1.80 cm-’ which is smaller than the B value of the 5p cornponentl6 ( B R = 2 04 cm-’) as expected from 1 uncoupling. The “unperturbed molecular parameters” of the L(8) level are A , = -60 crn-’, B, = 1.29 cm-l. When an interaction parameter of HQL = 9 cm-I is used, the calculated rotational levels fit the observed levels (MPI plus absorption data) within 10.5cm-’ except for J = 6 ] / ?for which there is an additional small avoided crossing (see above). Our interaction energy is about 24 times larger than the corresponding value calculated by Gallusser and Dressler.’ but a quantitative comparison would
-
-
+
( I 5) E. Miescher, unpublished results. (16) K. Dressler and E. Miescher, Astrophys. J . , 141, 1266 (1965) ( 1 7 ) Ch. Jungen and E. Miescher, Can. J . Phys., 46. 8987 (1968).
6880C
68700
68600 5
0
J
-3
IO
+
Figure 3. Plot of term values vs. J ( J 1) with tick marks showing values for the L(8) and Q(0) levels. Dotted lines correspond to the Q(O),F, component observed in vacuum-UV absorption (see text) of J -
be meaningless here since our local treatment can provide only effective parameters. More important are the mixing coefficients extracted from our deperturbation since they are related to the observed intensities. These calculated coefficients lead us to the following remarks: The observed intensity distribution among the four 211 components is reasonably well reproduced by our calculation by assuming that the ratio between the two vibronic transition moments from the A state is b L A / p Q A= 2 and that the ionization step is nearly saturated (see last section). The only deviation occurs for high J values of the Q ( 0 ) , F l L(8) level and will be discussed at the end of this paragraph. The L3j2level is almost not mixed with Q(0) and indeed appears with regular structure and line intensities in our spectra. On the other hand, the Llj2component crosses the Q(O),F, level around J = 11/2and the Q(0),F2 level around J = 6’/*, giving rise to mixing coefficients strongly varying with J . For J values above 6’/*, Q(0) is near to case b. As a consequence, we only see the Fi spin component from the pumped rotational levels of the A2Z+ state, i.e. the Fl(e) levels for most of the pump lines6 Both A-doubling components of this level are observed and well resolved (they are calculated from the transition energies of the Q lines and R + P lines for the f and e parity levels, respectively). For the lowest J values, the Q(0) level approaches case a because of the local interaction with Ll/2(8) and the additional interaction with B(28) and B(29). Therefore, nonvanishing intensities are predicted for rotational lines of the perturbed F2 spin component of Q(0). Indeed very weak lines corresponding to our calculated levels appear in spectra for J = 2’/*, 3 ’ / 2 , and 41/2and might be tentatively assigned to this component. Finally, our calculation predicts detectable line strength for transitions to the Q(O),F, Ll,2(8) level above J = 7’/* whereas no corresponding line appears in our spectra. This intensity anomaly may have several origins: (i) The small avoided crossing at J = 6 ’ / 2 observed in the L,,* Q,Fl level gives evidence of at least one other interacting level in this region in addition to the two ’II levels that we have considered here. Even if there is no measurable energy shift due to this small additional perturbation for other J values, intensities could be changed in a more significant way. From our data, we have not been able to characterize this additional level. (ii) The conditions of saturation of the
-
-
-
Excited States of NO
The Journal of Physical Chemistry, Vol. 90, No. 6, 1986
1089
TABLE I: Vacuum Wavenumbers for the L-A(8,l) and Q-A(0,l) Bands L-A( 8 , l )
N"
t,
PI
0 1 2 3 4 5
22051.7 22046.4 22039.7 22031.4
6 7
22010.5
8 9 10 11 12
21 997.7 21 983.9 2 1 968.3 22951.5 21 933.7
Q, 22 059.7 22058.2 22055.4 22051.2 22045.3
[L(8)4Q(0)I-A( 22063.5 22 066.2 22067.0 22066.8 22064.8 22061.5
1)
P2I
Q21
R21
22 124.3 22 116.5 22 112.5 22 107.1 22099.4
22 128.3 22 124.3 22 124.29 22 123.0 22 119.9 22 115.6
22 128.3 22 13 1.8 22 134.3 22134.9 22 134.0 22 132.8 22131.9
Rl
22029.5 22050.4 22081.9 22 080.9 22019.2 22042.8 22073.5 22007.7 22034.1 22065.9 21 994.8 22023.9 22058.1 21980.8 22012.9 22050.0 21 965.8 22000.8 22041.4
22 110.9 22108.4
22132.5
22 105.8 22 103.1 22099.9 22096.5 22091.3
22132.7 22 132.5 22 131.9 22129.9 22 127.2
a
I
22520
,
,
,
22470 cm-'
,
,
22420
Figure 4. Representative spectra of the K(2)-A(1) and F(3)-A(1) transitions corresponding to the following pump levels in the A state: (a) N" = I O ; (b) N" = 7; (c) N" = 6; (d) N" = 4; (e) N" = I . The lines marked with *, which appear for high rotational levels only, belong to the F(3)-A( I ) transition.
ionization step might not be fulfilled completely and therefore might modify the OODR-MPI intensities. This hypothesis is very unlikely since it would favor the observation of the Q,Fl-.L1,2 level even more for J values above the crossing since the ionization cross section is expected to be much larger for Rydberg levels than for the L valence levels. (iii) The transition moment pQA could present an effective variation with J due to interaction between Q(0) and B(28) and B(29). At this point it is meaningless to refine further our effective interaction treatment. Rather it would be more interesting to include our data in a general multistate interaction treatment (as has been done by Gallusser and Dressler for instance) in order to fully explain the observed intensities. 2. The K211( 2 ) F2A(3 ) B211( 2 9 ) B211(30)-A Zc(1) Transitions. The lowest rotational levels (J) < 1 1 1 / 2 ) of two Rydberg states K211(u=2) and F211(v=3) have been observed in vacuum-UV a b s o r p t i ~ n and ~ ~ ~emission," '~ respectively. The B211(u=29) level has also been observed in a b s o r p t i ~ n . ' ~We .'~ have extended the observation of the K(u=2) and F(u=3) levels up to J = 17'/* and have observed the lowest rotational levels of the B2n, u = 29 and u = 30 states ( J I The observation of higher J levels in the K(2) and F(3) states reveals an interaction between these two states. Figure 4 shows representative spectra of the K-A(2,l) and F-A(3,l) transitions. Line positions are listed in Tables I1 and 111. Just below and above these RydbergRydberg transitions, two weak transitions are observed which correspond to excitation of the B(u=29) and B(v=30) vibronic states. These spectra could only be obtained at relatively high probe laser powers such that the other transitions present in this region are saturated. Typical excitation spectra for B-A(29,l) are shown in Figure 5. Line positions are given in Table IV. The plot of the term values vs. J ( J + 1) for the B(29), K(2), and F(3) states (Figure 6) illustrates our MPI observations in this region. For low J values, the K 4pn, u = 2 Rydberg state mainly interacts with the valence B211, v = 29 state, as is known from
-
-
-
C
22420
22380
22340
cm" Figure 5. Representative spectra for the B(29)-A( 1 ) transition corresponding to the following pump levels in the A state: (a) N" = 4; (b) N" = 3; (c) N" = 1.
absorption studies.'* In our spectra the Q l lines of the perturbed B(29) level appear much weaker than the R, and PI lines, as shown in Figure 5. However, no significant corresponding intensity enhancement is observed in the Q , lines of the corresponding perturbed K(2) rotational levels (see Figure 4). The partial polarization of the laser excitation cannot explain the relative Q/R,P intensities observed here since such an effect would appear in all our OODR-MPI spectra which is not the case. For Jvalues above 6'/2, in addition to the K-A(2,l) lines, weak MPI lines of the F2A-A2ZC(3,1) transition appear in the spectrum. For J 3 6'/2 both Fl(e) and F2(f) parity levels are pumped via SIl(J) + R2,(J) overlapped two-photon lines. Nevertheless, the population ratio of the F,(e) parity vs. F2(f) parity is larger than 1 according to our observations for J = 71/2for instance (for which three different spectra were obtained by pumping 012(9'/2), S l l (18) A. Lagerqvist and E. Miescher, Can. J . Phys., 44, 1525 (1966).
1090 The Journal of Physical Chemistry, Vol. 90, No. 6 , 1986
Cheung et al.
TABLE 11: Vacuum Wavenumbers for the K-A(2.1) Band 0 1
2 3 4 5 6 7 8 9 10
22466.3 22459.5 22 451.6 22 443.3 22 434.1 22 424.0 22 41 3.4 22402.3
11
12 13 14 I5 16
22 474.2 22 411.2 22 467.6 22 462.9 22 457.2 22 450.9 22 443.6 22 435.9 22 428. I 22 420.2 22412.2 22 404.7 22 395.2 22 387.1 22 377.9 22 368.4
22 478.1 22 479.0 22 479.2 22 478.7 22 477.3 22 475.1 22 472.4 22 469.3 22 466.4 22 463.1 22 460.0 22 457.0 22 454.1 22 450.8 22 441.3 22 443.7
22483.1 22487.2 22 490.4 22 492.9 22 494.1 22 494.7 22 494.9 22 495.0 22 494.9 22 494.8 22495.1 22 493.4 22493.1 22492.1 22 490.2
TABLE 111: Vacuum Wavenumbers of the F-A(3,l) Band R’ ,v” P’ Q‘ 22482.5 6 22 485.5 7 22428.3 22 455.0 22 488.2 8 22423.6 22 454.0 22 490.7 9 22418.7 22452.8 IO 22 41 3.6 22451.5 22 492.7 11 22 408.0 22449.9 22 496.3 12 22 402.3 22 448.3 22498.3 13 22 398.0 22 446.4 22 500.2 14 22 392.2 22444.2 I5 22 386.2 22 439.6 16 22 380.3 22 432.4 TABLE IV: Vacuum Wavenumbers for the B-A(29,l) Band N” 0
PI
QI
22 362.5 22 857.6 22 350.4 22 340.5 22 329.3 22315.5 22 299.6 22 281.6
22 366.5 22 365.3 22 362.3 22356.7 22348.9 22 339.5 22 328.0 22310.6 22294.5 22 275.9
cm-‘
Rl 22 369.4 22 369.8
p2 1
22410.5 22364.9 22405.1 22358.6 22398.4 22 350.6 22 389.6 22 340.4 22 365.1 22310.3 22348.6 22 294.1
Q21
J
69040 ’
R2i 22 422.2
22418.4 22417.0 22414.2 22424.9 22409.4 22421.9 22 402.3 22 416.1 22 396.4 22 382.4
4
69300 -
69200
22495.0 22 490.4 22 485.0 22 479.7 22 474.8 22 470.4 22 466.4 22463.1 22 460.0
22 474.5 22465.1 22455.6 22 446.1 22431.1
22 506.9 22510.0 22 512.5 22 515.0 22518.0 22 521.5 22 525.’ 22 530.0 22 534.8 22 539.3
22 502.9 22 502.0 22 500.6 22 498.7 22 497.2 22 496.1 22495.0 22 494.9 22 494.8 22 495.1
1
.
.
.
#
,
0
/. 5
.
, IO
. J-L
.
. I5
,I
+
Figure 7. F(J) - 1.5J(J 1 ) vs. J ( J + 1) for the K(2) and F(3) levels observed by MPI. The deperturbation of e, levels of K2 and F3 is shown by -*.
+ R2,(5’/2) and Sz1(4*/2),respectively), and according to ref 20. Consequently, the strongest features of the spectra have been assigned to rotational lines probing the Fl(e) component of the A state (Tables I1 and 111),although some blended lines probably have a contribution from the other parity level of the A state for J 2 6 ] / 2 . For this reason both F1 and F, branches of F(3) are shown in Figure 6. Figure 7 shows, in an enlarged scale, the term values of the related parity levels of the K(u=2) and F(u=3) states. A local interaction between the e levels is observed around J = 11 and 12l/,, corresponding to a interaction matrix element smaller than 1 cm-’. No quantitative deperturbation has been attempted at this point. An indirect heterogeneous perturbation involving KZlI(u=2), F2A(u=3), and close-lying valence levels B211(u=29, ~ 3 0 and ) B’,A(u=8, u=9) could be at the origin of this weak interaction and could also explain the observation of the electronically forbidden F2A-A22+(3,l) transition. This last interpretation is correct only in the case of an isolated molecule and we have not experimentally eliminated the unlikely possibility that collisions are effecting these observations. At still higher J values, a small shift toward lower energies is observed in the K(2) level and is probably due to the interaction
-
IO J-% Figure 6. Plot of the term values vs. J ( J + 1 ) with tick marks showing values of J - I / 2 for the B(29), K(2), and F(3) (---) levels.
(19) Ch. Jungen, Can. J . Phys., 44, 3197 (1966). (20) J. B. Halpern, H. Zacharias, and R. Wallenstein, J . Mol. Spectrosc., 79, 1 (1980).
The Journal of Physical Chemistry, Vol. 90, No. 6, 1986 1091
Excited States of NO TABLE
V: Vacuum Wavenumbers for the B-A(30-1) Band PI
QI
22 845.4 22 840.0 22832.1 22 822.1 22 809.8
Rl 22851.8 22851.9 22 849.5 22845.2 22 838.4 22 829.9
p2
22 849.1 22 847.7 22 844.1 22 837.9 22 829.6 22819.0
22 778.7 22 760.1
22791.8 22 775.3
22 806.4 22791.8
22831.9
PI2
Nrr
22827.4
22 767.9
em-'
IQ-
N" 5
el
R21
22 907.7 22899.3 22 888.5 22 875.2
22915.5 22 91 1.1 22904.2 22 894.8 22 882.7
22919.4 22919.2 22916.0 22910.5 22902.3 22 892.0
22 840.9 22819.6
22 851.4 22831.7
22 863.3 22 845.0
)
,
,
I
1
I
I
I
1
7070C
A
1
I
R1
421
P2l
7060C
IQ4
70500
7040C
IQ-
3
7 0300 0
5
J
- v2
IO
+
Figure 9. Plot of the term values vs. J ( J 1) for the 5f(0) [---I, 4dZ-(1) [A-A],4dZ+(l)[A-A], C(8) [O-01, B(33) [O-01, and L(11) [@-e]levels observed in this work. Other unassigned transitions to levels in this energy region are not shown.
I
0
Rl Q1
1
.
23900
,
,
.
1
1
,
,
23850
,
1
1
I
,
,
23800
cm-' Figure 8. Representative spectra of the C(8)-A(I), 4d(l)-A(l), and 5f(O)-A( 1) transitions corresponding to the following pump levels in the A state: N" = 5 , "'4, Nr' = 3, N" = 0. See Tables VII, IX, and X for the line assignments.
with the upper lying B211, v = 30 valence level. Indeed the B-A(30,l) excitation spectrum has been observed in the 22750-22950 cm-' region from N " = 0 to N" = 8 and corresponding line positions are given in Table V. The F1 component of the B211state shows a smooth shift toward higher energies (the effective value of the rotational parameter B varies from 0.81 to 0.88 cm-l in the observed range) while the F2 component follows an apparently unperturbed variation with J in the same J range (effective rotation constant 0.70 cm-I). Unfortunately, we have not explored the higher rotational levels for which curve crossing between the B(30) and the K(2) levels must occur. From the low J rotational lines we have determined the experimental origins
of the two spin components of this valence level. These origins are given in Table VI and are in good agreement with Gallusser and Dressler predictions,' and with the recent MPI data of Ebata et al.4c Following Gallusser and Dressler the neaby L211, v = 9 level is expected to interact much more weakly with the Rydberg level K(2). This prediction seems consistent with the lack of observation of this valence level in our spectra. It is interesting to note that, in the absence of perturbations, all of the transitions in this region would be forbidden. 3. The B211(33)-0,0'4d(u,n) (1) -5flO) -C 3pr(8)-A2Z+(1) Transitions. The 23 670- through 24 090-cm-I portion of the excitation spectrum represents one of the most congested regions studied in this work in contrast, for instance, to the region of the 4f-A( 1,l) transition which occurs just be10w.~This region includes the allowed Av = 0, 4d(u,r)-A2Z+(l,l) transition in addition to other weaker Rydberg-Rydberg and valence-Rydberg transitions. In the corresponding absorption spectrum (between 70 300 and 70 700 cm-') t h e overlapping problems are still more severe. Nevertheless from low-temperature absorption spectra of the three isotopes l4Nl6O,14N1*0, and ''Nl6O, the 5f-X (0,O) band,21the Br-X (10,O) band,I9 and the L-X (1 1,O) band'* could be detected among many other band structures and were totally or partially analyzed. Thanks to these absorption data, we have been able to assign our OODR-MPI lines for l4Nl6O which yield complementary experimental information in this region. As seen in (21) Ch. Jungen and
E. Miescher, Can.J . Phys., 47,
1769 (1979).
1092 The Journal of Physical Chemistry, Vol. 90, No. 6, 1986
Cheung et al.
TABLE VI: Summarv of the Energies and Molecular Parameters for the *II Levels Observed in This Work" state c' Rb EOC EOd ECe L 8 312 68 604.4 68 539 L 8 68 669.2 68614 0 68 645.515 68 646 Q 0 'I2 68 638 Q B 29 68 907.4 68 907Is 68 892 B 29 'i2 68 963.1 68 962Is 68 930 K 2 I12 69019.0 69019.3'* 69 024 K 2 'I2 69047.8 69 04718 69 057 B 30 I12 69 390.2 69 387 B 30 'i2 69 460.3 69 4684c 69 459 C 8 I12 70 391.6 70 424 70 434.7f 70 453 C 8 'i2 B 33 70 567.5 70 563 B 33 'I2 70 629.0 70619 L 11 'i2 70 680.6 7067715 70772 L I1 !I2 70 860 W 0 'I2 70 749.3h 70 746.516 W 0 3 ~ 2 70 755.4 B 34 I12 70833.0 70 800 B 34 3/2 70905.8 70872 1 If2 71 002.4 71 002.716 71011 Q 1 'I2 71 029.4 71 039 Q B 35 71 086.9 71 086.516 71 040 B 35 3/2 71 147.9 71 219
Bd 1.25
0.91-0.80 I .24-0.94
0.8 1.16-0.96 1.29-1 . I 3 0.89-1.07 0.83-0.94 0.897
0.78-1.47 0.80-0.74 1.33-0.91 1.61-1.40 1.05-1.26 0.90-1 .OO
"Assignments to electronic states are somewhat arbitrary because of the extensive perturbations and are based upon the calculations of ref 9 except for B(29) and K(2) which have been labeled in accordance with the proximity of their respective branches. bThe label R is only given for convenience for the = component and J = 3 / 2 for the = (see text). 'Experimental energies observed in this work for the lowest rotational levels ( J = component). dExperimental energies from a b ~ o r p t i o n ' ~or~ OODR-MPI ~~~'~ work.4c The difference between our results and those from absorption studies must be due some systematic error in the calibration procedure. eCalculated energies from ref 9; note that the vibrational numbering of the L2 state has been correctedz3 from ref 9. For this reason, the numbering is different from that given in ref 4c. /Approximate rotational constants from the present data involving only the perturbed low J rotational levels ( J = 10.5). gRotational assignment uncertain. hValue for the lowest ( J = 'I2)level.
TABLE VII: Vacuum Wavenumbers for the C-A(8,l) Band N" 0 1 2 3 4 5 6 7 8 9 10 11
PI2
23 838.8 23 830.6 23 820.7 23 809.0 23 795.9 23 780.4 23 763.2 23 697.8
QII
PI I 23 846.8 23 842.2 23 836.5 23 828.9 23 819.3 23 808.1 23 795.6 23 779.5 23761.7 23741.1 23 717.5
23 850.7 23 850.2 23 848.1 23 844.6 23 839.3 23831.7 23 822.3 23 810.9 23 797.1 23 780.5 23 760.7
Rl I 23 854.2 23 856.1 24 856.2 23 854.9 23851.5 23 846.9 23 838.4 23 828.5 23815.8 23 800.1
23 758.8
p2 I
p22
R2l
Q21
23 893.8
23 861.9 23 852.0 23 828.2 23813.2 23 795.9 23 752.8
23 888.5 23 885.7 23881.7 23 875.8
23 893.8
23 859.3 23 848.5 23835.4 23819.7
23 880.0 23 870.7 23 858.9
23 776.2
TABLE VIII: Vacuum Wavenumbers for the B-A(33,l) Band *,,I
0 1 2 3 4 5 6 7 8 9 10
PI2
24014.8 24005.5 23 994.3 23981.1 23 965.4 23 948.3
PI,
Q1 I
24 022.7 24 01 7.5 24 010.1 24 000.8 23 989,4 24 975.8 23 960.9 23 943.8 23 926.2
24026.6 24 025.4 24021.9 24016.4 24009.0 23 999.7 23 988.2 23 975.7 23961.0 23 946.2
Rll 24029.2 24 029.8 24 028.2 24 024.7 24019.3 24 01 2.1 24003.1 23 993.1 23 98 1.4 23 968.9
the term value diagram of Figure 9, our data are stili incomplete on the 4d(~,7r),5f(O), and L( 11) levels. Our assignments are listed in Tables VII-X and will be commented on below without any attempt at deperturbation. In this section we will make a distinction between the I > 1 Rydberg states (4d and 5 f ) and the I = 1,3p7r Rydberg state. The former states experience much smaller Rydberg-valence interactions due to the nonpenetrating nature of the d and f orbitak2* On the other hand, the C 3pa(u=8) level is involved in a strong (22) Ch. Jungen, J . Chen. Phys., 53, 4168 (1970).
p22
24 064.5 24 052.9 24038.9 24 004.3
p2 1
24076.3 24 068.6 24058.9 24 046.6 24032.1 24 01 5.6 23 996.0 23 976.6
Q21
R2l
24 084.2 24 080.4 24 074.4 24 066.2 24055.8 24043.1 24028.5 24011.8 23 994.1 23 974.8
24088.1 24088.3 24086.2 24 08 1.9 24075 4 24066.8 24 056.0 24043.6 24029.3 24 014.1 23 997.7
TABLE IX: vacuum Wavenumbers for the 5 f - ~ ( 0 , 1 )Band N"
0 1 2 3 4 5 6 7 8 9
2R_, 23775.3
2Q-2
23 796.2 23 806.5 23 820.7 23 814.7 23 822.8
2P_3
OP-,
23 784.3 23 796.2 23 803.2 23 81 1.0 23 773.2 23 819.3 23 827.3 23 835.5 23 775.9 23 843.8 23 852.1
OQo
-2R3
23 157.2 23 776.7 23 748.8 23 776.3 23 740.8 23 776.3 23 777.0
The Journal of Physical Chemistry, Vol. 90, No. 6, 1986 1093
Excited States of NO
v
TABLE X: Vacuum Wavenumbers for the 4d(u,II)-A (1,l) Band
N"
Q-
R+
N" 0
5
0 1 2 3 4 5 6 7 8
v v
?
23 793.3 23 794.0 23 794.2 23 794.3 23 794.2 23 792.5
23 786.9 23 784.0 23 782.8 23781.7
multistate interaction with the nearby levels of the B211and LzII valence states. ( a ) ,II Rydberg-Valence Interaction. Typical spectra due to the ,II states in this region are shown in Figure 8 and Figure 9 displays the term value diagram of the observed levels. The energy origins given in Table VI are in good agreement with the predicted energies given in ref 9 for the B(33) state and only in fair agreement for the C(8) state. The C(8) state lies between the vibrational u = 32 and u = 33 levels of the B state and interacts with both levels. The calculated interaction parameters obtained ~~ by Gallusser and Dressler are 91 and 72 cm-l, r e s p e c t i ~ e l y . In addition, this state is involved significantly in a multistate interaction with L211(u=1l), B211(v=34), and W211(v=O).23These last three states are observed at shorter probe wavelengths and are presented in the next section. ( b ) 4d(u,~)--A(J,l)and 5f-A(0,1) Transitions. The 4d (u,T) u = 1 complex had not been positively assigned in earlier studies.24 The 3d ( u , T ) u = 0,125926as well as the 4d (a,*) v = 0 comp l e x e ~ ~ have ~ - , ~been observed in emisson and absorption studies. Because of the interference effects resulting from strong I uncoupling, many lines were missing. (Only P and R branches were observed for the 3dW-Z' and 3dZ+-Z+ subbands, respectively.) Furthermore, presumably due to predissociaton, neither the II+ nor the Z+ subbands were observed in the spectrum of the 4d ( U T ) u = 0 level. Among the emission s t ~ d i e some s ~ ~transitions ~ ~ ~ have a lower state as in our spectra. Therefore we expect to see the same type of structure which is dominated by a strong Q line. As expected, in our experiment the only strong feature is the Q line of the 4dB--A(l,l) excitation band. It is at the same position for all rotational quantum numbers, as is shown in Figure 8. The other lines are much weaker and their positions and intensities vary with N . Like the u = 0 level, no positive assignment can be made for the R(4dZ+-Zf) and the P(4dW-2+) branches. A few weak and broad features observed for 2 < N < 6 could be attributed to R(Z+-Z+) lines since their positions have the expected energies, assuming an I uncoupling parameter 7 = 2.1 cm-' and using eq 6 of ref 26. This assignment is only tentative and needs to be checked by further experimental investigation. Nevertheless this observation would confirm the hypothesis of a predissociation suggested by S ~ t e r . *The ~ C(v=8) and 4dZ-(v=I) energy levels cross each other around J = 8 ' / 2 . There is no evidence of a perturbation in this region. It would correspond to a "g-u" interaction between two Av # 0 Rydberg levels which is expected to be very small in N O . (See for example the very small interaction between 3p and 3s, u = 4 reported by Miescher.26 In the same energy region, a few lines belonging to the 5f-A(0,l) transition are observed. By comparison to our previous work on the 4f-A( 1, I ) transition6 the observed lines have been assigned to the -2R3,zP-3,OQo, and ,Q-, branches (with the branch labeling Rf-Nf'ICm,as in ref 6. The observation of the three most intense branches is consistent with a weak AD # 0 transition between two unperturbed Rydberg states. The energy origins = 70 308.5 cm-' and = 70317 cm-l are consistent with the corresponding origins for the I4N1*Oisotope given in ref 21. Many weak lines still remain unassigned in the OODR-MPI spectra (23) K. Dressler, private communication. (24) R. Suter, Can.J . Phys., 47, 882 (1969). (25) M. Huber, Helv. Phys. A , 37, 329 (1964). (26) E. Miescher, Can. J . Phys., 49, 2350 (1971).
0 0
0
V
0
v vv
? ?
V
1,
24 200 24 100 cm-l Figure 10. Representative spectra for the B(34)-A(1), W(0)-A(]), and L(l l)-A(I) transitions corresponding to the following pump levels in the A state: N" = 5, N" = 3, and N" = 1. V and v mark transitions to the F, and F2components of B(34); 0 symbols mark transitions to the W(0) level; 0 symbols mark transitions to the L(11) level.
24300
TABLE XI: Vacuum Wavenumbers for the L-A(l1,l) Band
N"
PI I
R,
0 ,I ~
I
~~
24 127.9 24 120.7 24111.3 24 099.4
24 135.9 24 132.5 24 127.0 24 119.2 24 109.3
24 139.7 24 140.5 24 138.7 24 135.0 24 129.0 24 120.8
24069.5 24051.4
24083.0 24 066.6
24098.1 24083.7
of this region and are not yet explained. Figure 9 displays the diagram of the levels assigned in our experiments in this energy range and Tables IX-X give the line positions of the transitions. 4. L211(l1 ) - unII(o)-B211(34)-A2Z;'(1) Transitions. Figure 10 shows typical A-state excitation spectra in the 24000- and 24 400-cm-I region. These OODR-MPI spectra provide complementary data to the absorption workI5 in which only one spin component was observed for each ,II state. As shown in this figure, the L-A(I 1,l) lines are very weak in comparison with the other transitions. Only the F, ( Q = 3/2) ( J < 9 1 / 2 )component could be identified. Line positions are given in Table XI and are in good agreement with previous absorption results.ls O n l y one spin component of the W(0) level is reported in the vacuum-UV absorption spectral6 and no experimental data have been previously reported on the B(34) (Table XIII) level. As shown in Table XII, the location of the W(O), F, spin component results from the observation of the Rzl lines only. Up to J = 7l/,, the assignments are straightforward since the only strong interactions are between the W(0) and B(34) levels. Note that the P, lines of the W-A transition are very weak, probably because of the Two-level mixing. For higher rotational levels the spectra become more complex and
1094 The Journal of Physical Chemistry, Vol. 90, No. 6, 1986
Cheung et al.
i
a /
d
b
1
I
C
d
I
TABLE XII: Vacuum Wavenumbers for the W-A(O.1) Band 0
24 208.4 24210.3 24212.3 24214.3 24215.2 24 21 5.5 24 214.3 24 206.6
1
2 3 4 5 6 7 8 9
24 190.9 24 185.1 24 178.8 24171.5 24162.8? 24 153.4 24 139.4
24 202.7 24 200.9 24 200.4 24 198.2 24 195.7 24 189.9 24 181.2 24 168.4
24214.5 24221.5 24 227.9 24 233.8 24 238.4 24 241 .O 24 239.7 24235.5
,
,
I
I
I
I
8
I
I
I
24400
24500 cm-l
Figure 12. Representative spectra for the B(35)-A(1) and Q ( l ) - A ( l ) transitions correspondingto the following pump levels in the A state: (a) N" = 8, (b) N" = 7, (c) N" = 5 , (d) N" = 3. A and A symbols mark transitions to the F, and F, components of Q(1); 0 and W symbols mark transitions to the F, and F2 components of B(35).
are not fully understood at present. The term values of these levels plotted in Figure 11 summarize our MPI observations. Tentative assignments of higher rotational levels are shown with dotted lines. Line positions for the W(0)-A( 1) and B(34)-A(1) transitions are given in Tables XI1 and XIII, respectively. 5 . Q211(I)-B211(.?5)-A2X+(1) Transitions. As shown in Figure 12, as in the energy region described in the previous section, we observe both spin components of the Q( 1 ) and B(35) levels while only the F, components were observed in
24 177.6
TABLE XIII: Vacuum Wavenumbers for the B-A(34,l) Band N" P,, PI 0, 0 24 292.1 1 24 288.4 24 290.4 24 286.3 2 24 280.2 24 282.6 24 280.2 3 24 270.7 24 274.9 24271.7 4 24 258.6 24 264.5 24 262.0 5 24 244.5 24 252.6 24251.8 6 24 228.7 24 239.7 24 242.3?? 7 24210.8 24 225.8 24 235.8 8 24215.1 24 229.9? 9 24 20S.5?
I
24600
+
Figure 11. Plot of the term values vs. J ( J 1) for the B(35), B(34), Q(I), and W(0) [---I levels. Dotted lines are based on tentative assignments of data at high rotational levels for W(0).
R, 24 294.4 24 294.3?? 24 292.1 24288.1 24 282.5 24 277.2 24 274.2 24 272.8?
p22
p21
421
R2I 24 364.9
24 341.4 24 329.4 24315.4 24 299.6
24353.1 24 345.4 24 334.9 24 322.4 24 307.8 24291.5 24 268.3? 24 247.5?
24 357.1 24 35 1 .O 24 342.4 24 328.2 24 316.0 24 299.9
24 352.7 24 357.9 24 350.7 24 342.5? 24 327.5 24315.1
24 263.6?
24 298.8
p21
Q21
R2l
24484.7 24485.0 24484.2 24 482.7 24 480.0 24475.9 24 469.6 24 461.3
24488.5 24 492.9 24 496.5 24498.7 24 499.8 24499.2 24 496.6 24491.3 24482.7
TABLE X I V Vacuum Wavenumbers for the 0-A(l.1) Band N"
pi2
24 450.0 24441.9 24432.9 24 422.3 24410.0 24 395.3 24378.0
PI
Q1
Rl
24 458.0 24454.3 24 448.7 24 442.0 24433.3 24 422.6 24 409.0 24 392.0
24461.5 24 461.9 24 460.7 24457.6 24453.2 24 446.4 24 437.1 24423.8 24 407.1
24465.9 24 468.7 24 469.1 24 468.7 24457.2 24 460.2 24451.2 24438.1 24421.3
p22
24464.7 24 457.2 24448.8 24439.6 24428.8 24416.9
24 477.0 24 473.0 24 468.7 24 463.4 24456.6 24 448.0 24 437.4
The Journal of Physical Chemistry, Vol. 90, No. 6, 1986 1095
Excited States of N O TABLE XV: Vacuum Wavenumbers for the B-A(35,1) Band N" 0 1 2 3 4 5 6 7 8
PI2
24 534.2 24 525.1 24514.2 24 501.6 24 487.6 24 472.9 24457.8
PI
QI
24 542.2 24 537.2 24 530.1 24521.6 24 51 1.7 24 501.4 24 490.5 24 479.5
24 546.0 24 545.0 24 541.8 24 537.1 24 530.9 24 524.0 24516.8 24 509.5 24 502.0
24 548.9 24547.1 24 544.6 24541.6 24 538.5 24 534.1 24 527.6
The spectra obtained in this region (Figure 12) appear to be more regular than those discussed in the last section. Branch assignments are thus relatively straightforward, and line positions are given in Tables XIV and XV. It is worth noting that in the same total energy region there are two other vibronic levels known from UV studies: 122+(7)27and GZZ-(9).28 Neither of these states, however, is observed in the present experiment which greatly simplifies the analysis. The F2 component of the Q ( l ) level is perturbed by other levels at higher energies, as evidenced by the variation of its rotational constant with J . It is most probably involved in a multistate interaction with the B(35) and K(3) levels. We have assigned the component at 71 148 cm-' to the F2 component of the B(35) level. Of course, the labeling of these perturbed levels is done for convenience only, with the arbitrary convention that the label refers to the greatest contribution in each perturbed level at the lowest J values. With this latter "assignment", a discrepancy appears with the Gallusser and Dressler calculations which predict a different ordering of the K(3) and B(35) states. Unfortunately, this spectral scan does not extend beyond the level observed at 71 148 cm-' and our data are not complete enough to give unambiguous assignments in this energy region.
Conclusion By the method of OODR-MPI we have explored the 69 80071 200-cm-' energy region of the NO spectrum which is especially congested and complex in the corresponding absorption spectrum. Table XI summarizes the experimental data obtained on zII states from the excitation spectra of the A, 3su, 2.Z+, u = 1 state. Due to the mixing with other valence and Rydberg levels, our results include not only the prominent npr, u = 1 states but also nd(u,a), ndb, and nf Rydberg states as well as several previously unreported vibronic levels of the B211 and L211 valence states. This new experimental information provides a good test for the multistate interaction model proposed by Gallusser and D r e ~ s l e r . ~ (27) E. Miescher, J . Mol. Spectrosc., 69,281 (1978). (28) A. Lofthus and E. Miescher, Can. J . Phys., 42, 848 (1964).
p2
Rl
P2I
Q2i
24 548.9
R21 24 607.0 24603.2
24 595.2 24 577.3 24 567.3 24 538.2
24 599.9 24 594.4 24 586.8 24 576.8 24 565.2 24 555.2
24602.5 24 596.8 24 589.2 24 580.6 24 572.4
This model yields a reasonably accurate description of the NO spectrum in the regions where sufficient experimental data were previously available but its predictions deviate significantly (up to 80 cm-' for some L levels) in energy regions where little or no experimental data were previously known. These new results indicate, for instance, that the W 2 n , 6pa state should have been included in the multistate interaction above 70 750 cm-I. It may not be realistic or advisable to extend this model by including more vibronic levels and thus more interaction parameters in order to describe these highly complex excited states of NO. In the region near the dissociation limit of the B and L states, and even above where predissociation occurs, such an approach seems cumbersome and in all cases incomplete beyond a certain level. An alternative way to describe this energy region is given by the multichannel quantum defect approach in which the valence states and their dissociation continua are included as new channels. Such an approach has already been used above the dissociation limit of the B and L state^.^^.^^^^' A generalization of this method including the discrete levels of the valence states may be the best way to interpret the detailed information provided by these OODR-MPI results. Acknowledgment. We are indebted to Professor E. Miescher and Professor K. Dressler for their continuous interest in our work and for providing useful unpublished data and finally for their pertinent comments on the manuscript. The data were obtained at the IBM Thomas J. Watson Research Center through an N S F industry, University cooperative research grant(CHE 802061 8) supporting W.Y.C., W.A.C., S.D.C., P.A., and J.J.W. The spectral interpretation was done in collaboration with D.G. through the support of a N A T O cooperative research grant. Registry No. NO, 10102-43-9. (29) A. Giusti, J . Phys. B, 13, 3867 (1980). (30) A. Giusti and Ch. Jungen, J . Chem. Phys., 80, 986 (1984). (31) M. Raoult, "Electronic and Atomic Collisions", Abstracts of Contributed Papers, Coggiola, D. L. Huestis, and R. P. Saxon, Ed., XIV ICPEAC, Palo Alto, 1985, p 40.
