8889
J. Phys. Chem. 1993,97, 8889-8894
Rotational Quantum Number and Vibronic Symmetry of NO2 Excited with Visible Light in the Region of 563-566 nm: Optical-Optical Double Resonance Measurement and Spin-Orbit Interaction between 2B2 and 2Az Vibronic States Kaon Aoki, Hidekazu Nagai, Kennosuke Hosbina, and Kazuhiko Shibuya' Department of Chemistry, Tokyo Institute of Technology, Ohokayama, Meguro-ku, Tokyo 152, Japan Received: February 5, 1993; I n Final Form: June 16, 1993.
An optical-optical double resonance (OODR) technique has been employed to determine the rotational quantum number and vibronic symmetry of NO2 in the region of 563-566 nm. The first excitation step corresponds to the intermediate 2Bz +%2A~(0,0,0)transition of a parallel type. The lowest rotational levels are identical with those of the 17 713.203-cm-l vibronic band reported by Delon et al. from their jet-cooled L I F experiment [J. Chem. Phys. 1991,95,5701]. The intermediate state is an admixture of two vibronic states, 2B2 (vibronicallymixed states between a1 vibrational levels on A2B2 and b2 vibrational levels on %?AI) and 2A2 (a1 vibrational levels on C2A2), being coupled through spin-orbit interaction. The second excitation step corresponds to the final 22B2(0,0,0) -intermediate 2A2transition of a perpendicular type, the analysis of which provides information on the C2A2state: the vibronic origin Tv 17 709 cm-l, the bond length r(N-0) 0.134 nm, and the bond angle B 109'. The vibronic level detected in this OODR study could be assigned to either C2A2(0,2,0) or C2A2(1,O,O).
-
-
Introduction The visible absorption spectrum of NO2 is well-known to be dense and extremely complex. The couplingsamong the electronic states (XZA1,A2Bz,&Bl, and C2A2)throughvariousperturbation forces (vibronic, spin-orbit, etc,) play important roles in causing the spectral irregularity. Fine and hyperfine structures due to electronic and nuclear spins partly contribute to its complexity. Some attempts have been successfully made to simplify the complex NO2 spectrum and to present spectroscopic evidence on the interstate coupling mechanisms. The vibronic coupling between A2B2 and high vibrational levels of the ground state is the major cause of the spectral complexity.l-' This vibronic coupling is so strong that if,has no means to distinguish two electronic states, A2B2 and X2A1. Therefore, in this paper we denote the vibronically mixed state as ZB2. In addition to the vibronic coupling, spin4rbit coupling has been considered as the mixing forceq5Definite experimental evidence of the spin-orbit interaction was presented by Brand and ChiuS6 In the region near 500 nm, they observed transition lines with the abnormal selection rules (AN = k2) in absorption and emission. The measurements of fluorescence lifetimes, g-factors?-9 line intensities,'O Stark coefficients?Jl and dipole m0ments~~J3 show J-dependent perturbation, which can be explained by the 2A~2Bz mixing through spin-orbit coupling.14 The excited $ate of @A2 is not directly accessible from the ground state (X2A1) through an electronic dipole transition. However, this dark state can partly contribute to the upper state responsible for the visible absorption spectrum in the spectral region shorter than 612 nm,ls because it can couple with ZBz by spin-orbit or Renner-Teller interaction. Our group has recently carried out the optical-optical double resonance (OODR) experiments in the 514- and 588-nm regions16J7and presented the conclusive evidence of the spin-orbit coupling between C2A2 and 2B2. In the present work, the OODR technique has been applied to the rotational and vibronic analysisof the absorption spectrum of NO2 in the region of 563-566 nm. It is concluded that the rovibronic levels arise from the mixing of dark @A2 and light ZB2 by spin-orbit coupling. The first excitation step is the 2B2 *Abstract published in Advance ACS Abstracrs, August 15, 1993.
0022-365419312097-8889$04.00/0
-
-gzA1 transition of a parallel type. The lowest rotational levels are identical with those reported by Delon et a1.26 from their jet-cooled LIF experiment. The second excitation step is the 22B2 transition of a perpendicular type, the analysis of which provides information on NO2 in the C2A2 state. The vibronic origin is determined to be about 17 709 cm-1, and the vibrational level could be assigned to either C2A2(0,2,0) or C2Az(1,0,O). The analysis of the K-type doubling gives the apparent geometry of r(N-0) 0.134 nm and 0 109O.
-
-
-
Experimental Section The experimentaldetails are describedelsewhere.18 and a brief description is given here. The output beam of a XeCl excimer laser (Lambda Physik EMG-103-E MSC) was split into two beams to excite pump (VI)and probe (YZ) dye lasers (Lambda Physik FL2002E and FL3001/2). The two dye lasers were fired withanopticaldelayofabout lOns toavoid thefirststepexcitation of NO2 by the probe laser. The spectral resolution of the probe laser was 0.3 cm-I (full width at half-maximum (fwhm)), which was comparable to thepredissociation line width of the 22B2 state of NO?. Using an intracavity etalon, the bandwidth of the pump laser wasreducedto0.04cm-1 (fwhm). Thefrequencycalibration of the pump and probe lasers was made by the laser induced fluorescence spectrum of I2 and the optogalvanic spectrum of Ne, respectively. The Occurrence of the double resonance was monitored by observing the UV fluorescence corresponding to the raeative transition from the final state (22Bz) to the ground state (X2A1)which was detected by a solar-blind photomultiplier (Hamamatsu R166) through an aqueous solution of NiS04 and a band-pass filter (Corning 7-54). The photomultiplier signals were recorded by a boxcar integrator (PAR1621 165) after being amplified by a preamplifier (PAR 115). The sample pressure was kept about 1 Torr to avoid collisional relaxation of the ~ 1 excited intermediate levels. In this experiment, two kinds of OODR excitation spectra were measured for a single intermediate rovibronic level. The u2-scanned spectrum was obtained by fixing u1, which reflects the rotational structure of the final 22Bz(O,O,O) state. Alternatively, the vl-scanned spectrum meas_ured by fixing ~2 reflects the rotational structureof the initial X2AI(O,O,O) state. Theconclusive rotational and vibronic assignment of the intermediate rovibronic Q 1993 American Chemical Society
8890 The Journal of Physical Chemistry, Vol. 97, No. 35, 1993
Aoki et al.
Figure 2. Schematic diagram of the UI-and u2-transitions observed in the spectrashown in Figure 1. The transition energies are expressed in units of cm-1.
17709
17710 v
17715
17716
(cm-l)
Figure 1. (a) u2-scanned spectrum measured by fUring U I at 17 709.36 cm-l. The same spectrum was obtained by fixing u1 at 17 715.26 cm-l. (b) ul-scanned spectrum measured by fixing u2 at 2 2419.5 cm-l.
level can be obtained independently from the two spectra. The rotational values of intermediate levels were calculated using the spectroscopic constants reported on XZA1(0,0,0)19 and 22B2(0,0,0)20and the u1- and v2-transition energies determined by the OODR excitation spectra.
Results and Discussion Rotational Analysis of Optical-Optical Double Resonance Spectra. The Occurrence of the double resonance was moni_tored by the ultraviolet fluorescence of NO2 22B2(0,0,0) X2A1. Typical OODR excitation spectra obtained in this experiment are shown in Figure 1, where spectra a and b correspond to the u2- and ul-scanned OODR spectra, respectively. The rotational quantum numbers determined from the v1- and q-transitions are defined as NK,J:(uI)and NK,&'(VZ),respectively. The vz-scanned spectrum (Figure la) was measured by setting v1 at one of the two peaks (17 709.36 and 17 715.26 cm-1) in Figure l b and scanning u2 over 22415-22425 cm-1. This spectrum (a) corresponds to the vz-transition from a single intermediate rovibronic level to the final 22B2 state. With use of the rotational structure of 22B2(0,0,0)reported,20the three lines can be assigned to the v2-transitions of a perpendicular type (Ma= f l ) terminating on the 31,2, 41.4, and 51.4 rotational levels of 22B2(O,O,O). The rotational quantum number of the intermediate level is thus determined as N~,&'(v2) = 40,4. Figure l b shows the ul-scanned spectrum, where the vz-value was fixed at 22 419.5 cm-I. Using the rotational constants for X2A1(0,0,0),19 we can assign the vl-transitions of a parallel type (NCa = 0) originating from the 40.4 and 20,~rotational levels of XZAI(O,O,O). The rotational quantum number of the intermediate level is thus determined as NK,X'(VI)= 30.3, which does not accord with NK,&'(Y~)determined above. The rotational assignmentsof the v1- and u2-transition bands in Figure 1 are illustrated by the diagram of Figure 2. It becomes clear that the single intermediate level has two rotational components. Table I lists all the VI- and v2-transitions and assignments for the K,' = 0 levels detected by the OODR method in the ul-region
-
of 17 670-17 737 cm-1. All of the intermediate levels have the rotational quantum numbers of "(VI) = odd, N'(v2) = even, and K,'(vl) = K,'(v~) = 0. The differencein the "quantum numbers is expressed as Ah' = N'(v2) - N'(u1) = f l , while all the K: values determined by both transitions are identical, AK,' = Ki(v2) - K,'(vl) = 0. The energies of single rovibronic levels (Em) derived independently from two relations, (Ei+ VI) and (Er- YZ), are in accord with each other within the experimentalaccuracies, 0.04 and 0.3 cm-I, respectively. All the ul- and v2-transition energiesand their assignments for the K,' = 1 levels are listed in Table 11. The band types of the u1- and v2-transitions are parallel and perpendicular,respectively, which are the same as those observed for the K,' 0 levels. The relation between the quantum numbers is also expressed as Ah'' = 0, f1and A&' = 0. In Figure 3 the reduced rovibronic energies ( E d ) are plotted against N'(vz)[N'(v~)+ 11. The asymmetric staggering might be noticed, although the data scatter: Namely, the upper and lower components are mainly composed of even and odd N'(v2) numbers, respectively. The spin assignments in Tables I and I1 are made based on the AJ = 0 selection rule. V i b d c Symmetry of the IntemediateState. Nitrogen dioxide belongs to a CZ,point group. According to nuclear statistics, the rovibronic symmetry of NO2 is restricted to A symmetry species or MA2). Therefore, the B ("B1 or "B2) and A (cyA1or "A2) vibronic states have only the rotational levels with odd and even N quantum numbers, respectively, in the KP)= 0 stack. All the intermediate levels of the K,' = 0 stack were measured to have odd N'(u1) quantum numbers in our OODR experiment, which means that the vibronic states of present interest are of B1 or B2 symmetry. Furthermore, the v1-transitions originatingwith the ground state ("Al) were measured to be of a parallel type. Thus, the spectroscopic analysis of the vl-transitions leads to a conclusion that the intermediate vibronic state has B2 symmetry. It is well-known that the dominant upper states of the visible absorption of NO2 are of "B2 symmetry, which are generated by a strongfibronic interaction between totally-symmetricvibrational levelson AZB2(YalWB2)and antisymmetrichigh vibrationallevels on the ground state (Vb2QpeA1). On the other hand, the v2-transitions are characterized by even N'(u2) quantum numbers in the K,' = 0 stack, a perpendicular type and the final-state symmetry of "Bz. The similar analysis of the u2-transitions. as described above on the vl-transitions, leads to another conclusion that the intermediate vibronic state has A2 symmetry. Because of nuclear spin statistics, one or the other of the asymmetry doublets in NO2 is missing in a given rotational level for K, = 1: Either the upper or the lower component of the K-type doublets is missing alternatively with N. As a result, the Nquantum numbers of the upper component are only even integer
NO2 Excited with Visible Light
The Journal of Physical Chemistry, Vol. 97, No. 35, 1993 8891
. ' = 0 Stack Measured in the 563-566-nm TABLE I: OODR Transitions and Assipnments for K intermbdiate level* 22B2(0,0,0) u2 (cm-1) Er- u2 (cm-1) NGx'(uz) NG&'(VI) Ei + vi (cm-9 N K , ~ Er (cm-9 224 16.7 177 17.6 40.4 FZ 30.3Fi 177 17.79 31,2 Fz 40134.3 22419.5 177 17.6 17717.80 41,4 FZ 40 137.1 22423.8 177 17.6 514 FZ 401 41.4 50,s FI 17724.30 22417.3 17724.1 60.6 Fz 51,r Fz 40141.4 22421.2 17724.1 17724.29 61,6 Fz 40 145.3 22427.5 17724.1 71,6 FZ 401 5 1.6 17726.3 60.6 F2 50.5 FI 17726.41 2241 5.1 51,4 FZ 40141.4 22419.0 17726.3 17726.40 61.6 Fz 40145.3 22425.3 17726.3 71,6F2 40151.6 2241 5.6 17736.0 80.8 Fz 70.7 FI 17736.05 71.6FZ 40 15 1.6 22420.6 17736.0 17736.07 81,s Fz 40156.6 22429.0 17736.0 91,s F2 40165.0 80.8 Fz 70,7Fi 17737.82 22413.9 17737.7 71.6FZ 401 5 1.6 22419.0 17737.7 17737.79 81,s Fz 40156.6 22427.4 17737.6 91,sFz 40165.0 100,io Fz 90.9FI 17750.07 22414.8 17750.2 91,sFz 40165.0 22420.8 17750.0 17750.08 101~0 F2 40170.8 lll,loF2 ll1,loFl 1213 FI 1 3 1 ~ FI 2
2243 1.6 22408.4 22415.0 22428.2
40181.5 40181.5 40188.0 40201.1
17749.9 17773.1 17773.0 17772.9
120.12 FI
130.13Fz
17773.06 17773.08
Range g2A1(0,0,0)
(cm-1)
Ei (cm-I)
17715.26 17709.36
2.53 8.44
177 15.86 17706.57
8.44 17.72
40,4F1 60.6 FI
17717.97 17708.68
8.44 17.72
40,4
17718.33 17705.70
17.72 30.37
613.6 F1 80.1 FI
17720.10 17707.42
17.72 30.37
60.6 FI 80,~F1
17719.70 17703.69
30.37 46.39
80,s Ft
17707.27 17684.54
65.79 88.54
12q1z Fz
~1
NG~" Fl 40,4F1
FI
604 FI
100.10 FI 16.14
Fz
a Rotational quantum numbers of an intermediate state independently determined by the u2 and V I transitions are indicated by Nw'(u2) and NK,&'(UI),respectively. Two components of electron spin fine structure (J = N l/2 and J = N - l/z) are denoted by FI and Fz, respectively. The rovibronic energy (Em) can be calculated by (Ei+ u1) and (Er- vz) with accuraciesof 0.04 and 0.3 cm-l, respectively. The rotational quantum number of NK,&'(YI) has odd N,even K., and odd Kc ('bl on "Bz), while N~,&'(uz) has even N,even Kc (ral on "Az).
+
"(I,
5
10
2)
15
20
i.
17715
177051
,
,
,
~
,
, 400
200
0 "(
2)["(
2)+11
Figure 3. Plot of the reduced rovibronic energies of K,' = 1 levels, Em - 0.414N'(u2)[N'(uz) + 11, against N'(vz)[N'(u~)+ 11. The data are taken from Table I1 and fit to eq 2 by using the least-squares method (two
solid lines). For the fitting, we eliminatedthree rotational levels of N'(uz) = 7 (upper), 9, and 1 1 (upper) as described in text. (0,2,4, ...) on the vibronic level with A symmetry ("A1 or "Az), while they are only odd (1,3,5, ...) on the vibronic level with B symmetry ("BI or "B2). Figure 3 might indicate that the N'(v2) numbers of upper and lower components are even and odd integers, respectively. The rotational symmetryof these intermediate levels is thus determined to be a2, which implies that the vibronic symmetry is A1 or A2. As shown in Figure la, the v24ransition terminating on "B2 is of a perpendicular t y p . Therefore, the intermediate vibronic state is concluded to have A2 symmetry, which accords with the conclusion derived from the data on the K,' = 0 stack. In the K,' = 1 stack, the vl-transitions originating with "A1 are of a parallel type, which implies the intermediate vibronic state is of Bz symmetry. As discussedabove, the symmetriesof thevibronic intermediate state are different, depending on whether it is assigned by the u1or v2-scanned spectrum. Namely, the intermediate rovibronic levels detected by OODR are admixtures of "B2 and "A2 states, and the molecular eigenfunction IME) can be written as
IME) = c1I"B2,NK) + c2IQA2,N'K') (1) where CIand cz are the mixing coefficients. The state component
-
expressed by the first term in eq 1 contributes to the vl-transition in OODR, which has been experimentally assigned to "Bz "AI. The second term component is responsible for the v2transition assigned to evB,+ "A2. In this energy region, a lot of al-vibrational levels of X2Al exist (one level every 10 cm-l).zl The 2A1 vibronic levels can also interact with the 2A2 vibronic state through spin-orbit interaction with the selection rules; N V = 0, f l and A& = f l . If the spin-orbit coupling between 2A1 and 2A2 occurs, abnormal transitions6 with AN = f 2 or an abnormal intensity patternZ2will be expected to be observed in the v2-scanned spectrum. This is not the case. Accordingly, the 2A1-2A2 coupling could not be recognized in our OODR experiment of this energy region. There are two candidates for the "A2 states detected by the v2-transitionsin the OODR method: symmetric vibrational levels on the C2A2state (ValWA2)and antisymmetricvibrational levels on the @B1 state (VbzWB1). The latter can be excluded, since the 22B~B2B1 transition is symmetry forbidden and the &B1g2A1transition is Franck-Condon forbiddenin the present energy region. On the other hand, the 22B&2A2 transition is symmetry allowed. Accordingly, the intermediate rovibronic levels belong to a certain vibrational level with al symmetry on @Az. The difference in the two sets of rotational quantum numbers determined by the V I - and u2-transitions suggests a mixing force acting on the zero-order vibronic states, "B2 and "A2. The selectionrules of spin-orbit couplingbetweenRA2andnB2vibronic states are AN= 0 (except K. = 0), f 1 and Ma = 0, which agrees with our observation. The "B2 and "A2 states can also interact with each other through Renner-Teller interaction, but the observation does not accord with the selection rule of RennerTeller interaction, AN = 0. Molecular and Geometric Parameters of @A*. Neglecting centrifugal distortion and other higher order terms, the rovibronic energy of a near-prolate top molecule can be written as23
+ (A, - &)Ka2 + @;N(N + 1) (2) where A,, B, [=(By+ C,)/2], and e; represent rotational constants. In the K,' = 1 stack, the effective B:m constants are approximated as (3B, + C,)/4 and (B, + 3C,)/4 for the upper EQr = T,
and lower components, respectively. In Figure 3 the reduced rovibronic energies are plotted against N'(u2) in the N'(N'+ 1)
8892
The Journal of Physical Chemistry, Vol. 97, No. 35, 1993
Aoki et al.
TABLE LI: OODR Transitions and Assignments for K: = 1 Stack Measured in 563-566-nm Range? 22Bz(0,0,0) N&&
a
Et (cm-') 40137.4 40147.3 40152.4 40157.0 40162.4 40 147.3 401 57.0 40162.4 40168.5 40 147.3 40157.0 40162.4 40168.5 40 147.3(.3)b 40160.3(.3)b 40162.3(.4)b 40 168.4(S)b 40175.4(.5)b 40160.4 40168.5 40 175.5 40 183.1 40 160.3 40176.4 40175.5 401 83.1 40191.7 40 176.4 40183.0 40191.6 40200.6 40176.4 40183.1 40191.7 40200.7 40176.4 40195.4 40191.7 40200.7 40211.1 40195.4 402 17.4 40211.1 40221.4 40233.6 40217.4 40221.4 40233.6 40245.2 40217.4 40242.4 40233.5 40245.1 40259.2 40217.4 40242.4 40233.6 40245.2 40259.2 40242.4 40270.3 40259.2 40272.0 40288.1 40270.3 40301.1 40288.1 40301.8 40320.1
intermediate level (cm-I) 22408.7 22418.6 22423.7 22428.4 22433.8 22414.8 22424.4 22429.9 22436.0 22409.4 22419.2 22424.6 22430.7 22405.2 22418.4 22420.3 22426.5 22433.4 22409.7 22418.0 22425.0 22432.5 22402.9 22419.0 22418.0 22425.6 22434.4 22413.1 22419.8 22428.5 22437.4 22409.9 22416.7 22425.2 22434.1 22399.2 22418.2 22414.6 22423.6 22434.0 22395.5 22417.6 22411.1 22421.6 22433.8 22411.0 22414.8 22427.0 22438.7 22392.9 22418.1 22409.0 22420.8 22434.7 22387.2 22412.2 22403.3 22414.9 22429.1 22384.5 22412.5 22401.2 22414.0 22430.1 22380.3 2241 1.2 22398.1 2241 1.8 22430.1
v2
Ef- v2 17728.7 17728.7 17728.7 17728.6 17728.6 17732.5 17732.6 17732.5 17732.5 17737.9 17737.8 17737.8 17737.8 17742.1(2.1)b 17741.9(1.9)b 17742.0(2.1)b 17741.9(2.0)b 17742.0(2. 17750.7 17750.5 17750.5 17750.6 17757.4 17757.4 17757.5 17757.5 17757.3 17763.3 17763.2 17763.1 17763.2 17766.5 17766.4 17766.5 17766.6 17777.2 17777.2 17777.1 17777.1 17777.1 17799.9 17799.8 17800.0 17799.8 17799.8 17806.4 17806.6 17806.6 17806.5 17824.5 17824.3 17824.5 17824.3 17824.5 17830.2 17830.2 17830.3 17830.3 17830.1 17857.9 17857.8 17858.0 17858.0 17858.0 17890.0 17889.9 17890.0 17890.0 17890.0
k2Al(0,0,0)
Ei + vI
NK,&'(V~) 61,s F2
51.4
17728.72 17728.70
vI (cm-I) 17712.57 17703.14
Ei (cm-I) 16.15 25.56
71,7 FZ
61.6 Fi
17732.66 17732.66
17712.59 17701.77
20.07 30.89
71,7 Fz
61.6 FI
17737.92 17737.93
17717.85 17707.04
20.07 30.89
81.7 F1 or F2
81.8
N&&'(vl) FI
F1 or F2
17742.10(.09)* 17711.21 17742.07(.08jb 17697.04
30.89(.88)b 45.03(.04)b
17750.59 17750.59
17719.70 17705.56
30.89 45.03
17757.45 17757.44 17757.48
17719.06 17712.41 17702.84
38.39 45.03 54.64
17763.23 17763.25
17700.71 17679.92
62.52 83.33
17766.40 17766.42
17721.37 17703.91
45.03 62.51
17777.20 17777.19
17722.56 17702.88
54.64 74.31
17799.93 17799.91
17725.62 17702.51
74.31 97.40
17806.50 17806.52 17806.49
17723.19 83.31 17709.12 97.40 17699.06 107.43
17824.29 17824.32
17700.39 17670.50
17830.24 17830.21 17830.23
17732.84 97.40 17722.78 107.43 17706.33 123.90
17857.87 17857.87
17733.97 123.90 17704.05 153.82
17889.87 17889.90
17736.05 17702.76
N&&"
123.90 153.82
153.82 187.14
Notation is the same as in Table I. b The values in parentheses are calculated as F2 sublevels.
scale. If two levels belong to zero-order states of "A2 and "B2, a pair of levels is expected to be observed as two components of the mixed state: One is a parent level mainly of "A2 character, and another is a daughter level of "Bz character. In fact, a pair
of levels are detected for N'(v2) = 7.1 1, and 16, as seen in Figure 3. The spin-orbit interaction is generally weak in polyatomic molecules,24 and the level shift due to perturbation will be small. Judging from the structure of K-type doubling in the K,' = 1
NO2 Excited with Visible Light
The Journal of Physical Chemistry, Vol. 97, No. 35, 1993 8893
TABLE IIk Comparison of Our OODR Data with the Jet-Cooled LIF Data for a 17 713-cm-l Band OODR'
jet-cooled LIFb
not observed not observed 17 717.79 (F1) not observed 17 724.29 (F1) 17 726.41 (F1) not observed not observed 17 736.05 (Fi) 17 737.81 (F1) 17 750.09 (F1) 17 773.05 (Fz)
17 714.000(Fl) 17 713.875 (F2) 17 717.795 (Fi) 17 717.660 (Fz) 17 724.289 (F# 17 726.407 (Fly 17 724.805 (F2) 17 725.600 (Fz) not observed not Observed not observed not observed
NK,A'(~I) 10.1
30.3
50,s
70,1 90.9 130.13 a
N'(
13 5 7 I r l,
II
1)
9
7
11
13
I I
17760-
I
Data are taken from Table I. b Data are taken from ref 26. C Jost,
R.(CNRS)Private communication. stack of wA2, we can classify the levels in Figure 3 into parent (mainly wA2) and daughter (mainly wB2) levels. For example, the lower levels with N'(uz) = 7 and 11 would be of A2 character, while the upper are of B2 character. One level detected as N'(v2) = 9 will be of B2 character as well. Two levels with N'(Yz) = 16 are supposed to be generated by mixing near resonant "A2 and w B in~ the zero-order approximation. Therefore, we eliminated three levels of BZcharacter (N'(u2) = 9 and the upper levels of N'(u2) = 7, 11) for the asymmetric doubling analysis. A leastsquares fit of the data on A2 levels to eq 2 is shown by two solid lines in Figure 3, which gives the rotational constants of B,' = 0.442 cm-1 and C,' = 0.382 cm-l. The geometric parameters of the @A2 state are then obtained as the bond length r(N-O) 0.1 34 nm and the bond angle 0 logo. These geometric values almost agree with the theoretical and other experimentaivalues. Gillispie et al. calculated the equilibrium geometry of C2Az as r(N-0) = 0.127 nm and e = 110°.Z5 Weaver et al. referred to the bond angle of NO2 in the C state and estimated it to be smaller than llOo, based on their photoelectron spectrum of N02-.lS From the recent OODR experiment in the 514-nm region,16these values are estimated as r(N-O) 0.14 nm and e 1020. Approximate vibronic band origin (T,) of the C2Az state is estimated to be around 17 709 cm-l from the-energies of seven levels listed in Table I. The T, values of CzA~(0,1,0)17 and C2Az(0,0,0)15 are reported to be 16 970 and 16 349 cm-l, respective1 Thevibronic level of the present study lies 739 cm-I above the J2A2(0,1,0) level and 1360cm-l above the CzAz(O,O,O) level. The ab initio calculationZ5gives the frequencies of 1360 cm-l for 01 (symmetric stretch) and 798 cm-l for wz (bend). At this moment, there are still two possibilities, (0,2,0) and (l,O,O), for the vibrational level of C2A2( Tv 17 709 cm-l) detected by the v2-transition in this OODR experiment. Comparison with Jet-Cooled LIF Data. In the preceding section, we have shown t h g a vibronic level located at 17 709 cm-1 is a mixed state of C2A2 and 2B2 coupled by spin-orbit interaction. In the present energy region, Delon et al. observed the lowest N' levels in the KL = 0 stack of two vibronic bands (the band origins: 17 700.080 and 17 713.203 cm-1) by the jetcooled LIF technique.26 Table I11 lists the rovibronic energies of all the K,' = 0 levels, which are taken from our OODR experiment (Table I) and the jet-cooled LIF experiment.26 The term values of 3,~,3(F1)and two 50,5(Fl) levels measured by the OODR experiment agree well with the values measured by the jet-cooled LIF experiment. It shows that a pair of interacting levels, So,s(Fl) on 2Bz and 60,6(F2) on C2A2, are detected by the OODR and jet-cooled LIF techniques. Using the molecular parameters (T, and B),one can estimate the approximate Em values of the higher "levels on the two 2 B ~ vibronic states. In Figures 4 and 5 , the E, values are plotted againstN'(ul)[N'(ul) 11 for the&'= Oand 1stacks,respectively. The solid lines of these figures present the extrapolated rotational
17700 0
"(
1)["(
1 y
d+11
Figure 4. Plot of the rovibronic energies of K,' = 0 levels against N'(v1)["(VI) + 11. Seven filled circles obtained from the data in OODR experiment (Table I). Eight open circles are obtained from the data in thejet-cooled LIF experiment.26Three filledcirclescannot be recognized in this figure because they are identical with the corresponding open circles as listed in Table 111. The two solid lines &_owthe extrapolated rotational structures of the 17 700.080-cm-1 band ( E = 0.400cm-1) and 17 713.203-cm-lband (B = 0.397 cm-I) reported in ref 26. "("1)
-
-
100
17g00F----?7
-
-
.
-
+
17700y
,
,
,
0
,
,
,
200 "(
IJ 1)["(
,
1 400
y
d+11
Figure 5. Plot of the rovibronic energies of K,' = 1 levels against N'(v1)[N'(vl) + 11. The data are taken from Table 11. The two solid lines show thecxtrapolatedrotationalstructurtsof 17 700.08kand 17 713.203-cm-1 bands reported in ref 26, assuming that (A - E ) = 2.4 cm-1. This assumption is based on our results that E: 5 0.442 cm-I and C,.' = 0.382 cm-1.
structures calculated for the 17 700.080 and 17 713.203 cm-1 bands measured by Delon et a1.26 The rovibronic energy values measured in this study are almost on the extrapolated lines. It turns out that the levels of "B2 measured by the jet-cooled LIF experimentare strongly perturbed by the levels belonging to @A2 and therefore could be detected as a mixed state of wA2and "B2 by the present OODR experiment.
Acknowledgment. The authors are grateful to Professor K. Obi for his encouragement throughout this work and Professor K. Kawaguchi (Nobeyama National ObseFatory) for the calculation of the rotational energies of NO2 X2A1(0,0,0) and 2ZB2(0,0,0). We also appreciate Dr. R. Jost for his critical comment on our results. This work was supported in part by the Grant-in-Aid for Scientific Research (No. 03453019) from the Ministry of Education, Science and Culture. References and Notes (1) Smalley,R.E.;Wharton, L.; Levy, D.H.J . Chcm. Phys. 1975,63, 4977.
8894 The Journal of Physical Chemistry, Vol. 97, No. 35, 1993 (2) Brand, J. C. D.; Chiu, P. H. J . Mol. Spectrosc. 1979, 75, 1. (3) Hallin, K.-E. J.; Mercr, A. J. J. Mol. Spectrosc. 1977,65, 75. (4) Pcrrin, A.; Camy-Peyret, C.; Hand, J. M. J. Mol. Spectrosc. 1981, 88, 237. (5) dilauro, C. J. Mol. Spectrosc. 1974, 51, 356. (6) Brand, J. C. D.; Chiu, P. H. J. Mol. Spectrosc. 1979,75, 1. (7) Persch, G.; Vcdder, H. J.; DemtrMer, W. Chem. Phys. 1986,105, 471. (8) Weber, H. G.;Bylicki, F. Acta Phys. Pol. 1986,A69, 699. (9) Bylicki, F.;Weber, H. G.; Zschccg, H.; Arnold, M.J. Chem. Phys. 1984,80, 1791. (10) Persch, G.; Vcdder, H. J.; DcmtrMer, W. J . Mol. Spectrosc. 1987, 123,356. (11) Bylicki, F.;Weber, H. G. Chem. Phys. 1982, 70, 299. (12) Brucat, P. J.; Zare, R.N. Mol. Phys. 1985, 55, 277. (13) Heitz,S.; Lampka, R.;Weidauer, D.; Htsc, A. J . Chem. Phys. 1991, 94,2532. (14) Bylicki, F.;Weber, H.G.;Persch, G.; DemtrMer, W.J. Chem. Phys. 1988,88.3532.
Aoki et al. (15) Weaver,A.;Mctz,R.B.;Bradforth,S.E.;Neumark,D.M.J.Chem. Phys. 1989,90,2070. (16) Shibuya, K.; Kusumoto, T.; Nagai, H.; Obi, K. J . Chem.Phys. 1991,
95, 720. (17) Nagai, H.; Aoki, K.; Kusumoto, T.; Shibuya, K.; Obi, K. J. Phys. Chem. 1991,95, 2718. (18) Shibuya, K.; Kusumoto, T.; Nagai, H.; Obi, K. Chem. Phys. Lett. 1988, 152, 129. (19) Bowman, W. C.; DeLucia, F. C. J. Chem. Phys. 1982,77, 92. (20) Hallin, K.-E. J.; Merer, A. J. Can. J. Phys. 1976,54, 1157. (21) Delon, A.; Jost, R. J . Chem. Phys. 1991,95, 5686. (22) Nanai. H.: Shibuva. K.: Obi. K. J. Chem. Phvs. 1990. 93. 7656. (23) He;zberg,' G.Mhecular Spectra and Molecular Structure III. Electronic Spectra and Electronic Structure of Polyatomic Molecules.; Van Nostrand hinccton, NJ, 1967. (24) Hallin, K.-E. J.; Merer, A. J. J . Mol. Spectrosc. 1977,65,163. (25) Gillispie, G.D.;Khan, A. U.; Wahl, A. C.; Hostney, R.P.; Krauss, M. J. Chem. Phys. 1975.63, 3425. (26) Delon, A.; Jost, R.; Lombardi, M. J. Chem. Phys. 1991,95,5701.
