Electronic spectra and single rotational level fluorescence lifetimes of

J . Phys. Chem. 1985,89, 1391-1395 The desorption spectra for D2 coadsorbed with oxygen also point to strong repulsive interactions in that a drop in the D2 thermal desorption maximum is noted with increasing oxygen coverage. The subsequent alteration of the Dz desorption peak also implies a site blocking effect engendered by the coadsorbed oxygen, an effect that is also apparent in the (20-0 coadsorption experiments. In summary, we have presented data which shows significant alterations in the thermal desorption characteristics of C O or D2 adsorbed on C- or 0-covered Os(OOl), and we have explained our results by invoking repulsive lateral interactions.I8 (17) Madey, T. E.; Engelhardt, H. A.; Menzel, D. Surf. Sci. 1975,48,304.

1391

Acknowledgment. The authors thank Professor Thor Rhodin for the loan of the Os(OO1) crystal used in these studies and the National Science Foundation for their financial support under Contract No. CHE80-00038. In addition, the helpful suggestions and criticisms concering this manuscript of Dr. P. N . Ross, Dr. W. T. Tysoe, and Dr. A. Stacey are gratefully acknowledged; Professor W. H. Weinberg is thanked for supplying us with advance information concerning his Ru(001) studies. Registry No. CO, 630-08-0; H2,1333-74-0; Os, 7440-04-2; C, 744044-0; 02,7782-44-7. (18) Further studies of Os(OO1) chemistry are detailed in Shanahan, K. L. Ph.D. Thesis, University of California-Berkeley, 1984.

-

Electronic Spectra and Single Rotational Level Fluorescence Lifetimes of Jet-Cooled H,CO: The i 1 A 2 EIAl 2:6:, 2:4:, and 2:4:6: Eric C. Apel and Edward K. C. Lee* Department of Chemistry, University of California, Imine, California 9271 7 (Received: October 1 , 1984)

The weakly absorbing trawitions of 2;6; and 2;4;6; near the strongly absorbing 2;4: transition were favorably detected by laser fluorescence excitation spectroscopy of jet-cooled H 2 C 0 (So),because of the higher fluorescence yields of the 2I6l and 2'4I6l levels. The vibronic origins ( y o ) and rotational constants of the 2l6I and 2'4I6l levels of H 2 C 0 (SI)have been determined as follows: 2l6I: 5o = 30252.21 cm-'; A' = 9.204 cm-'; B' = 1.108 cm-'; C' = 0.996 cm-'; 2l4l6I: to= 30395.00 cm-I; A ' = 8.918 cm-'; B'= 1.105 cm-I; C'= 0.993 crn-'. The S1lifetimes have been measured for a number of SRL's of 2I6I (Edb E 2064 cm-') and they vary in the range of 20-90 ns. This behavior indicates that a moderate degree of the lumpy continuum characteristic is observed even at EvibE 2000 cm-'. The SI lifetimes of 2143and 2l4I6' were 1 1 5 and 20-30 ns, respectively.

Introduction Recent interest in intramolecular dynamics of small polyatomic molecules has led us to study the role of rotation in mixing the vibrational states of the SI state of H2C0.'v2 Since the Coriolis interaction increases with the increase in rotational quantum numbers J'and K', the rotation-induced mixing of the zero-order excited vibrational states becomes quite important for the highlying rotational levels populated a t room temperature, e.g., in H2CO (so)? HzCO SI),^^,^*' and C6H6 (SI)? This effect should be nearly negligible for the low-lying rotational levels populated in a supersonic jet, and therefore a study of intramolecular dynamics free (or greatly reduced) of the rotation-induced effect can be made with jet-cooled molecules. Our firs: attempt for such a study was made recently with the HzCO A1A2 XIA1 5; transition and the preliminary results were reported earlier.' We report here the fluorescence lifetime measurements of low-lying rotational levels of 2l6l as well as the fluorescence excitation spectra of 2;6& 2;4:, and 2;4;6;. The region of the H2C0 2;4: vibronic transition was chosen for our study in order to characterize the vibronic levels pertinent to Coriolis interaction. At room temperature, we probed the spectral region around the 2A4: absorption band by laser-induced

-

~

~~~~

(1) (a) N. L. Garland and E. K. C. Lee, Faraday Discuss., Chem. SOC., 75, 377 (1983); (B) Chem. Phys. Lett., 101, 573 (1983). (2) E. C. Ape1 and E. K. C. Lee, J . Phys. Chem., 88, 1283 (1984). Dai, J. Chem. (3) (a) P. H. Vaccaro, J. L. Kinsey, R. W. Field, and H.-L. Phys., 78,3659 (1983); (b) D. E. Reisner, R. W. Field, J. L. Kinsey, and H.-L. Dai, J . Chem. Phys., 80, 5968 (1984), and references therein. (4) V. Sethuraman, V. A. Job, and K. K. Innes, J . Mol. Spectrosc., 33, 189 (1970). (5) (a) D. A. Ramsay and S. M. Till, Can. J. Phys., SI, 1224 (1979); (b) C. M. L. Kerr, D. C. Moule, and D. A. Ramsay, Can. J . Phys., 61.6 (1983). (6) (a) E. Riedle, H. J. Neusser, and E. W. Schlag, J . Phys. Chem., 86, 4847 (1982); (b) E. Riedle and H. J. Neusser, J . Chem. Phys., 80, 4686 (1984). (7) N. L. Garland, E. C. Apel, and E. K. C. Lee, Chem. Phys. Lerr., 95, 209 (1983).

0022-3654/85/2089-1391$01.50/0

fluorescence. Sethuraman et al.4 and Kerr et aLsb have shown the rotational levels of this band to be perturbed by at least two other "unseen" levels. The most likely candidates (on the S1 manifold) responsible for the perturbation in terms of energy difference and symmetry are the 3l6l, 2'6l, and 2I4I6' levels. We found recently in our wavelength-resolved fluorescence emission studies at room temperature that these latter two levels are Coriolis coupled not only to the 2143level but also to each other by a b-axis Coriolis coupling.2 However, the 2;6; band had remained undetected until now and the 2;4;6; band8 received little attention because of combined experimental problems associated with very weak transition intensities and spectral congestion. The advantages of applying the supersonic jet/laser-induced fluorescence technique to these band systems are clear: the jet-cooled spectrum is much less congested than the room temperature spectrum, the entire oscillator strength of a vibronic band is distributed over a few rotational levels near the vibronic origin, and the fluorescence intensity of the weaker absorbing band with a relatively high fluorescence quantum yield is detected with a higher sensitivity. As a result, the vibronic origin can be found easily with the appropriate determination of the rotational constants. This information can then be used to elucidate more extensively spectroscopic data and perturbations present in high-lying rotational levels populated a t room temperature.

Experimental Section The supersonic jet apparatus has been fully described elsewhere: so only the essential features will be outlined here. A Nd:YAG pumped dye laser (Quanta-Ray, DCR-l/PDL-1) was used as the excitation source. For the broad bandwidth experiments (fwhm N 1 cm-I in the UV) the laser was continuously (8) V. Sethuraman, Ph.D. Thesis, Vanderbilt University, 1968. The vibronic origin of 30 396 cm-' for the 2 w 6 1 band was given. (9) M. Noble, E. C. Apel, and E. K. C. Lee, J . Chem. Phys., 78, 2219 (1 983).

0 1985 American Chemical Society

1392 The Journal of Physical Chemistry, Vol. 89,No. 8,1985 H,CO

(~'Az--xlAl)

,

Ape1 and Lee

1

Po= I a t m A r , D = 0 . 2 0 m m

2; 6;

t

t lb I

u.

U

-

2 " " ' " L l l l 30400 30350

I

1

I

I

30300

8

i

1

I

I

Z

I

I

'

'

I

30284

1

I

I

I

I

30280

8

4

30276

I

/

,

'

I

I

I

30272

,

I

I

1

30268

,

I

I

30264

l

30250

1

(cm-')

Figure 1. A low-resolution (fwhm N 1 cm-I), fluorescence excitation spectrum of jet-cooled H 2 C 0 in the 3285-3310-A region. The electronic 24 ;,: and 26;.; The band transitions shown are A1A2 RIAl 24;6;,; origins are indicated by broken lines.

-

scanned by angle tuning the grating with a stepping motor. For the narrow bandwidth experiments (fwhm I 0.08 an-*) the laser was continuously scanned by pressurizing the gas-tight chamber containing the grating and intracavity etalon with SF6 gas. A UV output of 1-3 mJ per pulse at 10 Hz was obtained after the frequency-doublingof the visible output near 640 nm using DCM dye. The visible wavelength of the dye laser was calibrated against I2 absorption lineslo at room temperature, and we feel that the calibration is good to -0.02 cm-'. The fluorescence signal was collected at right angles to the incident laser beam which crossed the free jet. It was focused onto a high-gain photomultiplier tube (EM1 9863 QB) with a 7-cm focal length lens through a laser blocking filter (Corning 4-96, cutoff at X I345 nm). For the fluorescence excitation experiments, the signal from the photomultiplier was processed with a gated boxcar averager (PARC 162/165). A typical gate width of 50 ns was used. For the time-resolved experiments, a 100-MHz waveform recorder (Biomation 8 100) interfaced to a signal-averaging system (Tracor Northern TN 1500/Nova 3) was used. In a typical measurement 2000 shots were averaged. The formaldehyde monomer was introduced into the gas stream (at 1.0-1.5 atm) by heating the polymer placed inside of the stagnation chamber 2-3 mm before the nozzle. The nozzle diameter was 0.20 mm, and the laser crossed the free jet at 4 mm from the nozzle. Ar was used as the carrier gas for all of the experiments as the S / N ratio was poor when helium was used. A complex set of experimental parameters involved in the thermal decomposition of the H 2 C 0 polymer in the camer gas stream and the free jet expansion does not permit us to explain the poor S/N ratio obtained with helium.

Results The low-resolution fluorescence excitation spectrum of the jet-cooled HzCO in the 2i4; region is shown in Figure 1. Since these transitions may be subject to saturation, an interpretation of the observed intensity variation must be made with caution and hence is qualitative. In rem ition of this l i t a t i o n , we can state that the 2b6; band and the 2 r i band have an approximately equal value of the product, oscillator strength X fluorescence quantum yield, whereas the 2i4& band has an order of magnitude lower value. The rotationally resolved, fluorescence excitation spectra for the three close-lying vibronic bands are shown in Figures 2-4. The rotational transitions are labeled by the asymmetric rotor notation, ux"K,rtJ~( J ). The identification of the various rotational lines of the 2040band was made in accordance with the previous assignments of Sethuraman et aL4 There are only two unassigned lines in each of the 2i6b and 2b4: bands. However,

'5

(10) S. Gerstenkom and P. Luc, 'Atlas of the Absorption Spectrum of I2 cm-l)-, CNRS Publications, Paris, 1978.

(14,800-20,000

,I

(H,CO i n A r )

1

1

30264

1

1

1

-

1

1

30260

1

,

,

1

30256

1

,

,

1

30252

1

1

,

,

1

30248

cox(cm-9

,

,

,

30244

Figure 2. A high-resolution, fluorescence excitation spectrum of jetcooled HICO in the 2;6: region with the vibronic origin at uo = 30252.21 cm-'. The bandwidth of the laser is -0.08 cm-I (fwhm) and the rotational temperature is estimated to be - 4 K. Note that the sensitivity is reduced by a factor of 2 for the 30273-30285-cm-I region. Two unidentified lines are at 30245.01 and 30246.25 cm-' (see text).

t

1

1

1

30364

lb

1

1

1

1

1

30360

1

1

1

1

30356

1

1

1

1

30352

1

1

30348

i-4

PP,, I I )

I

,

IO348

,

,

I

,

30344

,

,

I

,

,

30340

,

I

,

,

,

30336

I

,

,

30332

,

30: 8

Cox(cm-9

Figure 3. A high-resolution excitation spectrum of jet-cooled H 2 C 0 in 2;4: region with the vibronic origin at uo = 30340.055 cm-l. The bandwidth of the laser is -0.08 cm-' (fwhm) and the rotational temperature is estimated to be -4 K. The rotational transitions were identified with the assignment by Sethuraman et al. (ref 4); two observed lines at 30337.23 and 30350.66 cm-' do not fit to the assignment (see text). 244b66 (H,CO

in Arl

Po = 1.5atm, D=0.30mm

30400

30396

30392

3038E

iiox(cm-I) Figure 4. A high-resolution, fluorescence excitation spectrum of jetcooled H 2 C 0 in the 2;4:68 region with the vibronic origin at yo = 30395.00 cm-I. The bandwidth of the laser is 0.08 cm-I (fwhm) and the rotational temperature is estimated to be -4 K.

such a thorough check cannot be made for the 24;6;; because of the poor SIN ratio.

transition,

The Journal of Physical Chemistry, Vol. 89, No. 8, 1985

Fluorescence Lifetimes of Jet-Cooled HzCO TABLE I: SRL Lifetimes (78) of the 2’6l Level (Edb= 2064 cm-’) of H2C0 Cooled in the Free-Jet Expansion with 1 atm of Ar Carrier Gas at 20 Nozzle Diameter Distance ~~~

1393

TABLE II: Rotational Constants (cm-I) and Vibronic Origins (em-’) of H2C0 (SI)2’6l, 2’4’, and 2l4’6’ Levels’ level 2I6l 2l4’ 214161 b.d

~

excitation transn

upper state rot Q N

‘Rl I(2) %2(2) %o(l) ‘RI 1(1) ‘R02(2) ‘ROl( 1) ‘RodO) ’PIl(2) ’Q12(2) ’Qii(1) PPlO(1)

321 322 220 221 312 21 I 110 303 202 101

E,,,’

000

cm-I

7F,b

45.2 45.2 38.9 38.9 21.1 14.6 10.3 12.6 6.3 2.1 0

53.5 68.9 65.4 90.5 70.1 39.5 54.0 54.6 68.0 56.0 20

-

ns f 5.0 f 3.8 f 1.9

f 2.3 f 1.0 f 3.0 f 1.8 f 2.3 f 1.3 f 3.0

-

‘E,,, was calculated from the rotational constants given in Table 11. bThe nonradiative decay rate (knJ is for all SRL’s studied here, since 7F 5 0 . 0 1 ~ ~ .

The fluorescence decays were measured for a large number of single rotational levels (SRL‘s) and found to be single exponential ) the SRL’s of in most cases. The fluorescence lifetimes ( T ~ of the 2;4: system excited by ‘R,(O), ‘Rlo(l), ‘Rll(l), and PPl0(l) were too short to be measured, Le., < 15 ns. The SRL‘s of 2I4’6l excited byqQlo(l), qQll(l), andqRlo(l) had values of T F 20 ns with extremely weak signal. The values of T~ measured for the SRL’s of the 2h6; system are summarized in Table I.

Discussion 2W Band. The fluorescence excitation spectrum of the 2;4: band in Figure 3 indicates that it has a rotational line intensity distribution of a B-type band expected at T N 4 K for each nuclear spin isomer (ortho for K,” = odd and para for K / = even). The rotational temperature was calculated on the assumption that the fluorescence excitation intensity is proportional to the absorption intensity, Le., the fluorescence yields from all rotational levels of a given vibronic level are equal. In previous studies of the laser fluorescence spectroscopy of jet-cooled H2CO,I2J3it was found that there was no ortho-para conversion during the jet expansion from the source chamber at room temperature, and the population ratio of the ortho to the para isomer in the jet was -3:l. The present result shows a similar ortho:para ratio. Of the 19 rotational lines shown in Figure 2, 17 lines are observed to be in agreement with the transition frequencies reported by Sethuramaan et aL4 and they have been accordingly assigned. Two unassigned lines in Figure 2 appear at 30337.23 and 30350.66 cm-’. The 30 337.23-cm-’ line appears at 0.36 cm-I to the red of the line assigned to PRI2(2) by Sethuraman et al.4 and 0.28 cm-’ to the red of the line for PRI2(2) calculated by the asymmetric rotor program using the rotational constants and the 2h4: band origin given in Table 11. The -0.30-cm-’ displacement for this line could be due to a perturbation of unknown origin but its relative intensity is in agreement with the calculated value at T N 4 K. Therefore, we believe that the 30337.23-cm-’ line in Figure 3 must be PRI2(2). The K,’ = 0 upper state of the 2h4: PRI2(2) transition cannot be perturbed by the K,’ = 0 level of 2l6I by a-axis Coriolis coupling or by any rotational level of the higher-lying 2I4l6l by c-axis Coriolis coupling. We were also unable to assign the 30 350.66-cm-’ line to any transition with the relative intensity consistent with T = 4 K. W e find that the intensity of ‘Rlz(2) is about a factor of 2-3 too low compared to the intensity of ‘RI1(2) which depopulates the higher-lying asymmetry doublet. Since the fluorescence lifetimes (7F)of all of the 2143rotational levels were too short to be measured and hence we have no measurements of the relative values of the fluorescence quantum yield (OF),we (1 1) V. A. Job, V.Sethuraman, and K. K. Innes, J . Mol. Spectrosc., 30, 365 (1969). (12) (a) H. L. Selzle and E. W. Schlag, Chem. Phys., 43, 111 (1979); (b) W. E. Henke, H. L. Selzle, T. R.Hayes, E. W. Schlag, and S.H. Lin, J . Chem. Phys., 16, 1327 (1982). (13) N. L. Garland, E. C. Apel, and E. K.C. Lee, Chem. Phys. Letf.,95, 209 (1983).

origin Etvib

A’

B’ C!

30252.21 (5) 2064 9.204 (8) 1.108 (8) 0.996 (10)

30340.055 2152 8.2162 1.10529 1.01097

30395.00 (10) 2207 8.918 (7) 1.105 (13) 0.993 (13)

‘The numbers in parentheses represent one standard deviation. The ground-state rotational constants (cm-I) given in ref 5b were used: A” = 9.4062826; B” = 1.2612422; C” = 1.1078991; U t K= 6.5040 X lo4; DfJK = 4.1945 X lo”; U’J= 2.3858 X lo4. *The centrifugal distor-

tion constants (cm-I) of the upper state were arbitrarily chosen to be the same as those of 2l4’ in ref 5b. (UK= 3.85 X lo4; UJK= 5.62 X U J= 3.32 X lo“.) CThedata of ref 11 have been recently improved upon in ref 5b. dThis band was identified to be at 30396 cm-’ and of an A type (ref 8). Our earlier estimate of the vibronic origin obtained from the room temperature fluorescence excitation data for the qR5(J”) subband was too low, -30370 cm-I (ref 2). TABLE III: Assignments of the Observed Rotational Transitions in the 2:6: Band obsd transn transn AV v.-, cm-l assinnmt (calcd-obsd).‘ cm-’ 30 282.29 ‘R12(2) 0.08 30 28 1.79 ‘Rll(2) 0.03 30 280.61 ‘RId1) 0.04 30 280.45 ,RIO(l) 0.03 30 275.92 ‘Q12(2) -0.08 30275.43 ‘Qii(2) -0.10 30266.1 1 ‘R02(2) -0.03 30 264.46 ‘ROl( 1) 0.02 30262.55 ‘R00(0) 0.03 30 260.02 ‘Qoi(1) 0.01 30259.26 ‘Q02(2) -0.02 30 249.1 1 PR, I (2) 0.02 30 247.81 PRlO(1) 0.05 30246.25 ? 30245.01 ? 30 243.79 ‘QI I (1) 0.02 30 243.31 ’Q12(2) 0.02 30 242.59 ‘QI 3(3) 0.08 30241.52 PPlO( 1) 0.02

‘Transition frequencies were calculated with the asymmetric rotor program for the vibronic origin of 30252.21 cm-’ and the rotational constants given in Table 11. are unable to give a definitive explanation of the above intensity variation. It should be noted that the jet-cooled SRL‘s of 43 (Evib = 948 cm-’) have SI lifetimes in the range of 14-52 ns with a median value of 20 ns.Izb Therefore, an S,lifetime shorter than 15 ns for SRL’s is not surprising. 2h6; Band. The fluorescence excitation spectrum of the 2;6: band, shown in Figure 2, was analyzed as a C-type band as expected from the vibronic symmetries involved in the transition. This was based on the previous observation that 6; band is located and furthermore an a-type at -80 cm-’ to the red of 4: Coriolis interaction has been found between 43 and 61.5a The spectrum for a C-type band at T N 4 K (for each ortho-para nuclear spin isomer) calculated from the rotational constants and the vibronic origin given in Table I1 gave a good fit to the observed spectrum in Figure 2. The rotational assignment as well as the agreement between the observed and the calculated frequencies are shown in Table 111. Again, 17 of the 19 observed rotational lines shown in Figure 2 have been identified, but we were unable to assign the 30 245.01- and 30 246.25-cm-I lines to any transition with the relative intensity consistent with T = 4 K. Both of these unidentified lines are too strong to be ‘P0,(5) (expected at 30245.73 cm-’) . The relative intensity of PPlo(1) is too low by a factor of 3 compared to many of the other lines. It should be noted from Table I that the observed value of 7F N 20 ns for the SRL excited by PPlo(l) is 3-4 times shorter than the values of 7 F (in the 50-70-11s range) for most of the other SRL‘s. On the assumption

-

The Journal of Physical Chemistry, Vol. 89, No. 8, 1985

Ape1 and Lee

that the radiative lifetimes ( 7 R ) of all “unperturbed“ SRL’s are the same for a given vibronic level, the low value of the fluorescence excitation intensity of PPI,,(1) can be rationalized on the basis of the lower value of 7 F (= @F7R) observed for the Ow level prepared. If a value of 7 R N 6 ps for the 2I6I level is assumed as for the 4’ level,lZbJ4J5the value of aF= 0.01 can be estimated for typical, low-lying SRL’s of 2I6l and aF N 0.003 for the Ow level of 2I6l. The unusually low values of 7 F and aF(for the Ooo level) and IF (for the pPlo( 1) line) are consistent. The erratic variation of the observed values of SRL 73 for 2’6l as a function of J‘and K,‘ is reminiscent of the behavior observed for the 4l level, but the magnitude of variation for 2I6l (EvlbN 2064 cm-I) is considerably less than that for 4I (En,,’ = 125 cm-I). The range of the observed variation of nonradiative rates (knr) for the SRL‘s of 2l6l (a factor of 3 as shown in Table I) is reduced by 1 order of magnitude from that observed for 4’ in a previous jet study12band by 2 orders of magnitude from that observed for 4, in previous room temperature studiesI4J5where a greater range of J’and K,’ levels was populated. If the lumpy continuum of the isoenergetic So levels is responsible for the erratic behavior of the nonradiative lifetimes of 2l6l as well as 4’, the above values should be a measure of the “lumpiness”. Since the variation in the nonradiative transition rate (knr)for the jet-cooled SRL’s of 2I6I is smaller by a factor of than that for the jet-cooled SRL’s of 4’, the variation in the average value of ( E , - E,) for 2’6I (located at -2000 cm-I above 4’) should be smaller by a factor of 10 than that for 4’, where E, and E , are zero-order energies of the prepared SIand the coupled Solevels, respecti~ely.’~ This is a surprisingly small change for Enbof -2000 cm-I. Again, an accidental resonance in the lumpy continuum is probably responsible for the short lifetime of the Ow of 2’6l. It is interesting to note also that the Ow level does not have the longest value of rF among the SRL‘s of another vibronic state, e.g., 43. It is interesting to compare the rotational constants of 2143and 2l6’ listed in Table I1 to those of 4261(A’ = 8.9022; B’ = 1.1225; C’=1.0130 cm-I),I6 43 (A’= 8.5384; B’= 1.11774; C’= 1.01839 cm-’),I6 and 6, (A’= 8.935; B’= 1.124; C’= 1.004 crn-’).IZb The values of A’determined for the low-lying rotational levels of 6’ and 2’6I by jet spectroscopy are similar to that of 4261but are surprisingly high compared to those of 43 and 2143. If an unusually large positiue value of AK can be confirmed in 6’ and 2I6l, it should be a clear indication of the perturbation due to the a-axis Coriolis coupling with 43 and 2143which are observed to have negative AK valuesI6 at room t e m p e r a t ~ r e . ~ Henke et a1.Izbhave recently studied the jet-cooled SRL’s of 6’ and 43. The vibronic origins of 6’ and 43 are 29084 and 29 135.909 cm-I, respectively, giving an energy gap (A&) of 52 cm-’ which is considerably less than the energy gap of 88 cm-I between 2I6l and 2143 (see Table 11). Therefore, the extent of the a-type Coriolis interaction between 2’6l and 2143 ( A J = 0; AK, = 0; AK, = f l ) should be weaker than that between 6’ and 43on the basis of hE,b values. It is interesting to note (see Figure 7 of ref 12b) that the fluorescence intensities of the r-form subband are 4-5 times stronger than the pform subband for the 4; transition whereas the r-form subband intensities are 4-5 times weaker than the p-form subband intensity for the 6; transition. No such intensity bias has been observed in our jet/laser fluorescence spectrum of the 2;6; and 2;4; transitions (see Figures 2 and 3). However, we have observed a similar rotational line intensity bias in the room temperature emission spectra from 5’ and 1’4, levels with AE,,, = 2.8 cm-l,1a*2and Sethuraman et al.4 have observed a similar rotational line intensity bias in the 5; and lk4; absorptions at room temperature. Here, a strong a-type Coriolis interaction is responsible for the observed intensity bias. With the jet-cooled rotational levels, Coriolis interaction should be minimal, and hence it would be surprising to find that the r-form/p-form intensity

TABLE I V Assignment of the Observed Rotational Transitions in the

1394

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(14) J. C.Weisshaar and C. B. Moore, J . Chem. Phys., 70, 5135 (1979); 72, 5414 (1980). (15) K. Shibuya, P. W. Fairchild, and E. K. C. Lee, J . Chem. Phys., 75, 3397 (1981). (16) D. J. Clouthier and D. A. Ramsay, Annu. Reu. Phys. Chem.,34, 31 (1983).

2:4:6:

Band obsd transn ueq,cm-I 30 400.41 30 400.22 30 400.02 30 398.93 30 398.75 30 398.56 30 397.15 30 394.56 30394.28 30 389.68 30389.33

transn

assignmt

Aw (calcd-obsd),” cm-’ -0.07 -0.01 0.00 0.01 0.09 0.08 0.06 0.06 0.04 -0.13 -0.13

a Transition frequencies were calculated with the asymmetric rotor program for the vibronic origin of 30 395.00 cm-I and the rotational constants given in Table I f .

bias observed with the jet-cooled rotational levels of 6l and 43 is due to a-type Coriolis interaction. An attempt to establish firmly the origin of this rotational intensity bias is certainly in order. 2;4;6; Band. The fluorescence excitation spectrum of 2;4;6; band in Figure 4 was analyzed as an A-type band. This was based on our observation2 and the previous observations by Sethuraman* at room temperature that the 2;4;6; band is located at 40-60 cm-I to the blue of the 2;4: band. Because this band is expected to have a very low absorption intensity, Le., the oscillator strength is -0.01 of that for the 41/43 band,I6J7it is not surprising that the S / N ratio observed for the rotational lines in Figure 4 is quite poor. As stated in the Results section, it was difficult to measure precisely the lifetimes; however, they were in the -20-ns range, a factor of 2-3 shorter than those observed for the 2l6’ SRL’s. Again, the spectrum at T = 4 K calculated by using the rotational constants and the vibronic origin given in Table I1 gives a good fit to the observed spectrum which is shown Figure 4. The rotational assignment and the agreement between the observed and the calculated frequencies are shown in Table IV. All of the 11 observed rotational lines are identified. Our preliminary results2 on the Coriolis-induced transition of the 2;4;6; 9R5(J’3 subband at 296 K are consistent with the present jet spectroscopic results on the qR1(J’l subband transition at -4 K observed here. The former transition has an order of magnitude greater excitation intensity than the latter transition, for which little or no intensity borrowing is possible for the low values of J’ and K,’ by the Coriolis interaction mechanism. It is interesting to note that the term difference for Aui = 1 between 2I4]6I and 2I6l levels is 142.8 cm-I, whereas that between 4I and 4O levels is 124.5 cm-l. Mbronic Absorption Intensities. The A1A2 RIA, electronic transition of formaldehyde is vibronically induced by either the bl (u4) or the b, (v5;vg) vibratiocal mc$e.I4 Strickler and Barnhart (SB)” have shown that in the A X transition of H2C0 -66% of the intensity is carried by the vq/ band (B type, shared nearly equally between 4’ and 43 progressions) and the remaining intensity is divided among others as follows: 26% by u5 (C type), 6% by v6 (C type), 0.5% by each of the combinations u4v5 (A type) and Y4v6 (A type), and -1% by the magnetic-dipole allowed component. van Dijk et al. (VKKB)I8 have obtained the vibronic transition intensities by an ab initio C I calculation and the oscillator strengths are 8.7 X 10“ for 2i4: and 1.84 X 10” for 2A6: They did not calculate the oscillator strengths of the A-type bands so that the estimate for 2;4;6’ is not available. The observed fluorescence intensities of the 206i P and 2A4; bands in Figure 1 are about equal. Clearly the ratio of these observed values is in clear disagreement, by an order of magnitude, with the absorption intensity ratios predicted, Le., 0.2: 1.O by SB1’ and 0.2: 1.O by VKKB.IB However, the observed fluorescence intensity of 1 ( 2 ; 4 ~ 6 ~ ) / 1 ( 2=~ 0.2:l.O 6 ~ ) is in reasonable agreement with the

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(17) S. J. StricMer and R. J. Barnhart, J . Phys. Chem., 86, 448 (1982). (18) J. M.F. van Dijk, M. J. H. Kemp, J. H. M., Kerp, and H. M. Buck, J . Chem. Phys., 69, 2453 (1978).

J. Phys. Chem. 1985,89, 1395-1401 absorption ratios of 0.1:l.O estimated by SB." Since fluorescence intensity (ZF)is proportional to the product of absorption intensity (I,) and fluorescence quantum yield (aF)in the limit of Beer's law, the above difference may be rationalized if the condition for @F(2143)N 0.2@F(2161) 0.4@F(214161)is met. This estimate for the values of aFis also consistent with the values of T F measured within the accuracy of the measurements involved in determining T F , ZF, and I,. Vibrational Mode Selectivity. Jet-cooled S R L experiments should allow a more clear-cut examination of the vibrational mode selectivity for nonradiative processes, since little or no rotationinduced effect should obscure the observation of purely vibronic processes for the low-lying rotational levels. Indeed, an appreciable l/rF,if rR >> T!) differentiation in the nonradiative rates (k, of rotationally cold molecules is observed among nearly isoenergetic

1395

SVL's, 2'6l, 2143,and 2'4161; for example, an ensemble average value of rFis -3 times longer for 2'6' than for 2143. Since the previous measurements of k,, for many SVL's (with high-lying rotational 1evel~)'~JO did not include 2'6' and 2'4'6l levels, a direct comparison with the present measurement is not yet possible.

Acknowledgment. We thank Dr. D. A. Ramsay for improving the numerical treatment of our rotational analysis and for valuable discussion on the comparison with the 6; and 4; bands. This research has been supported by the National Science Foundation under Grant CHE-82-17121. Registry No. H,CO, 50-00-0. (19) (a) E. S. Yeung and C. B. Moore, J. Chem. Phys., 58,3988 (1973); 60,2139 (1974); (b) see J. C. Weisshaar and C. B. Moore, Annu. Rev. Phys. Chem., 34 (1983), and references therein. (20) (a) R. G. Miller and E. K. C. Lee, J. Chem. Phys., 68,4448 (1978);

(b) see E. K. C. Lee and G. L. Loper in "Radiationless Transitions", S. H. Lin, Ed., Academic, New York, 1980, p 1.

Electrodeposited CdS/SAIPc Heterojunctlon Cell: Otlgln of the p-Type Semiconducting Character of the Surfactant Aluminum Phthalocyanine M. F. Lawrence and J. P. Dodelet* ZNRS-Energie, C. P. 1020, Varennes, Qulbec, Canada, JOL 2PO (Received: July 10, 1984)

An ITO/CdS/SAlPc/Au solar cell made by sequential electrodeposition of cadmium sulfide and a surfactant aluminum phthalocyanine (SAlPc) has been analyzed. Electrodeposited CdS, being a nearly degenerate semiconductor, acts as a blocking contact and all the band bending occurs in the phthalocyanine. The p-type semiconducting character of SAlPc is due to "doping" by photogenerated electrons and the subsequent behavior of SAlPc in the dark is related to the energy, Et, at which the electrons are trapped above the valence band. The capacitance discharge technique is used to determine the barrier parameters Vo,wo, and N . The same technique can also be utilized as a means of monitoring the rate of electron detrapping when the organic material returns to its equilibrium trap occupancy in the dark. This permits the evaluation of E, 0.55 eV as well as the existence of a Gaussian distribution of valence and conduction band levels about E, and E, with a 0 of -0.08 eV. The results are used to produce a complete energy band diagram of the cell in the dark (immediately after illumination).

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Introduction The crucial influence of trapping states on the electrical properties of insulators and semiconductors has been thoroughly emphasized over the past few decades. During this period, major efforts have been made to ascertain the nature of these localized states and determine important physical parameters such as their density and their energetic distribution within the forbidden band gap. The earlier studies dealing with this subject were based upon the theory of carrier injection into an insulator first given by Mott and Gurney.' Rose,2 and later Rose and L a m ~ e r tdeveloped ,~ the classical theory of space charge limited currents (SCLC) that was subsequently applied to the field of organic crystals by Helfrich and Mark.4 Since then SCLC flow, as a diagnostic means for determining the characteristics of trapping sites, has been employed extensively to study dark and photoconductivity of organic compounds showing some aptitudes for photovoltaic energy conversion or for possible use in electronic

Carrier trapping sites in organic solids display either a continuous or discrete energy spectrum within the band gap. The discrete traps are generally related to chemical impurities and those forming a quasi-continuous distribution are associated to the statistical dispersion of lattice imperfections causing local variations in the polarization energy of charge There are at present two important distribution functions used to characterize the dispersion of trap energies in the forbidden energy gap. One is the exponential distribution proposed by Rose2 and the other is a Gaussian distribution, favored by Silinsh." In most cases involving organic crystals, it has been difficult to distinguish by SCLC measurements between the Gaussian and exponential distributions because they have almost identical current-voltage For this reason, many experimenters in the past have preferred to use the exponential distribution instead of the more sophisticated Gaussian approach. While the exponential distribution may be a good approximation, there is evidence that the actual distribution of traps in the forbidden zone may be

(1) Mott, N. F.; Gurney, R.W. 'Electronic Processes in Ionic Crystals"; Oxford University Press: Oxford, 1948; 2nd ed. (2) Rose, A. Phys. Rev. 1955, 97, 1538. (3) Rose, A.; Lampert, M. A. Phys. Rev. 1959, 113, 1227. (4) Helfrich, W.; Mark, P. Z . Phys. 1962, 168, 495. (5) Sussman, A. J. Appl. Phys. 1967, 38, 2738, 2748. (6) Heilmeier, G. H.; Warfield, G. J . Chem. Phys. 1963, 38, 163.

(7) Hamann, C. Phys. Status Solidi 1968, 26, 31 1. (8) Owen, G. P.; Sworakowski, J.; Thomas, J. M.; Williams, D. F.; Williams, J. 0. J. Chem. SOC.,Faraday Trans. 2 1974, 70, 853. (9) Loutfy, R. 0. Phys. Status Solidi A 1981, 65, 659. (10) Sworakowski, J. Mol. Crysr. Liq. Crysr. 1970, 11, 1. (11) Silinsh, E. A. Phys. Status Solidi A 1970, 3, 817. (12) Lanyon, H. P. D. Phys. Rev. 1963, 130, 134.

0022-3654/85/2089-1395$01.50/00 1985 American Chemical Society