Fluorescence emission spectroscopy, radiative lifetimes, and


Shibuya et al. Fluorescence Emission Spectroscopy, Radiative Lifetimes, and Vibrational Relaxation. Rates of the 4' and 4' Single Vibronic Levels of D...
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940

The Journal of Physical Chemistry, Vo/. 83,

No. 8, 1979

Shibuya et al.

Fluorescence Emission Spectroscopy, Radiative Lifetimes, and Vibrational Relaxation 'Ap) Rates of the 4' and 4' Single Vibronic Levels of DPCO and HDCO (i Kazuhiko Shibuya, Dennis L. Holtermann, Jon R. Peacock, and Edward K. C. Lee* Department of Chemistry, University of California, Irvine, California 92717 (Received September 15, 1978) Publication costs assisted by the U.S.Department of Energy

Vibronic level-to-level intensity distributions of the fluorescence emission from the ground (4O) and the first excited (4l) vibrational levels of D2C0and HDCO (A IA2) have been determined in the wavelength range 36&490 nm. From the intensity distribution data obtained at Boltzmann equilibrium under high buffer gas pressures of n-C4Hlo,the ratio of the radiative lifetimes for the two SVL's, T ~ ( 4 ~ ) / 7 ~ ( 4 ' )has , been determined to be 0.82 f 0.08 for D2C0 and 0.72 f 0.15 for HDCO. The collision-induced4l- 4O vibrational relaxation by the parent molecule (2) proceeds with rate constants (klO,p) of 9.4 x 10-l' for D2CO and 17 X 10-I' cm3 molecule-l s-l for HDCO, exceeding the gas kinetic rate by a factor of 3 and 5, respectively.

Introduction The radiative lifetimespf the 4O and 4l single vibronic levels (SVL's) of H 2 C 0 (A 'A2), T'R and T R , respectively, have been determined recently from the fluorescence intensity distributions observed for the level-to-level(LTL) vibronic transitions a t pressures where collision-induced vibrational relaxation of the laser-pumped 4' level populates the 4O level.' We have extended the above study to the other isotopic species, D2C0 and HDCO, as a part of the systematic study of the emission spectroscopy1and molecular energy transfer2 involving excited formaldehyde molecules in the gas phase. Vibrational levels of the three isotopic formaldehyde molecules of interest as well as the Boltzmann factor, N(E,b), a t room temperature are shown for comparison in Figure 1. The ratio of the radiative lifetime:, TR(4l)/T'R(4'), was found to be 0.70 f 0.10 for H 2 C 0 (A lA2)in an earlier study.l This ratio is found to be 0.82 f 0.08 for D2C0 and 0.72 f 0.15 for HDCO in the present study. From the values of this ratio and the TR(4') values reported in the literature, the ~ ( 4 ' )values for the ground vibrational levels of D2C0 and HDCO are obtained. The collisional vibrational relaxation rates of the 4'- 4O conversion have also been measured. The details are given below. Experimental Section The preparation and the use of D2C0 and HDCO have been described earliera3 The flash lamp pumped dye laser (Chromatix CMX-4) and the optical multichannel analyzer (Princeton Applied Research Models 1205KD and 1207) were used in the optical configuration as described in the previous study1 for measuring the fluorescence emission spectra of H2C0. In the previous study,l the diffraction grating was set for recording only the first position spectrum in the 360-460-nm region. In the present study, the diffraction grating was rotated to a new setting for recording the second position spectrum in the 400-500-nm region, -40 nm to the red of the first position. Therefore, it was possible to investigate a slightly wider spectral region this time. With the second position spectrum, weak intensity peaks in the 430-490-nm region were observed with somewhat higher sensitivity than with the first position spectrum. The 4; transition was excited at -353 nm with a 0.07-nm bandwidth and a t 23 "C. The first position spectra are shown in Figure 2 to illustrate the resolution observed for the vibronic transitions and the effect of pressure on the intensity distribution. n-C4Hlo was used 0022-3654/79/2083-0940$0 1.OO/O

as a vibrationally efficient but electronically poor collision partner.

Results D2C0. The procedure by which the level-to-level (LTL) emission intensities observed a t various pressures can be appropriately assigned to the 4l and 4O levels in H2C0 (A) was described previously.1,2 We have used the same procedure in the present-study. The emission intensity distributions of D2C0 (A), obtained by 352.5-nm laser excitation of the 4; transition, are shown in Table I. The observed LTL intensities were normalized to the 378.0-nm band (peak 2), since this band can be assigned to the unresolved 4; and 2: 4; transitions and is more importantly free of contamination from the emission bands originating from the 4O levels populated through collisional pEocesses. A pure SVL emission spectrum of the 4' DPCO (A) taken in low temperature Xe matrix (20 K)4 shows clearly that peaks 1,3,5,and 7 originate from the collisionally pumped 4O level, consistent with the pressure dependent behavior of the LTL intensities in Table I. Peaks 2, 4, 6, and 8 belong to the laser pumped 4l level. The emission bands from the 4l and 4O levels of D2C0 (A) are conveniently separated from each other a t regular intervals (see Figure 2 ) . The SVL emission intensity distributions of DzCO (4O and 4') which best fit the data in Table I are given in Table 11, together with the LTL transition assignments. The low temperature matrix spectrum4 indicates that the 4' LTL transition is about a factor of 2 as intense as the 2: 4! LTL transition, and our lower resolution spectrum confirms this (see Figure 2). The less intense transitions in the overlapped peaks are listed in parentheses next to the more intense transitions in Table 11. It has been shown previously' that in order to evaluate the ratio of the radiative rates, hR(4')/kk(4'), it is necessary to obtain the ratio of the LTL emission intensities, i(4i)/i(47), originating from the 4' and 4O levels a t Boltzmann equilibrium, which can be achieved a t high n-C4Hlopressures:

Note that i(4;) in eq 1represents the intensity of peak 2 consisting of two components, 43 and 2: 4;, and Nl/No is the Boltzmann population ratio, exp(48.5 cm-llkr). The value of i(4i)/i(4?)decreases with the increasing pressure 0 1979 American

Chemical Society

Vibronic Levels of D,CO and HDCO

The Journal of FfIySicaf Chemistfy. Vol. 83. No. 8, 1979 941

TABLE I: Observed Level-to-Level Fluorescence Emission from D,CO Excited at 352.5 nm (Corrected for Soectral ResDonse) run no. D,CO. mtorr

peak no.

Aem.

16 6

nm

4 9

3 15 17 25 50 99 i,bd(rel to the int at 378.0 nm)

366.0 0.32 0.32 0.45 0.53 0.58 2 378.0 1 1 1 1 1 3 392.4 1.07 1.02 1.48 1.74 1.93 4 405.6 2.39 2.32 2.41 2.33 2.47 5 -421 1.92 2-15 2 48 1.49 1.32 ~. 6 -436 2.63 2.50 2.42 2.29 2.38 7 -455 1.03 0.87 1.26 1.43 1.52 8 -470 Intensities from the second position spectrum, normalized to the average i o b d values of peaks 1

~

1 148

S57" 280

0.61 1 1.96 2.31

2.21

2 -.36 ..

R 34

2.25 1.57

2.56 3.03 1.68

4 and 6.

TABLE 11: SVL Fluorescence Emission Intensities and Transition Assignment from 352.5-nm Excitation of D,CO 4 ' level

peak no. 1 2 3 4 5 6 7 8

nm

4" lewl

transitions

1.

366.0 378.0 392.4 (389) 405.6 ( 4 0 4 ) 423 (421,418) -436 -455 -469

transitions

i,

4:

1.00

1.00

4: ( 2 ~ 4 3

2.37

i

0.06

4: (2p4:; 2:4;)"

2.41

5

0.14

2p4: (4;; 2:4:)O

G1.7 x i e G 7.48

2:4:; 2p4:; 2!4: f

0.5b

3.27

%

0.06

4: (2:4p)"

4.21

*

0.27

40 (2:4:; 2:4p)"

2.77

f

0.23

274; (2:4!,

L i p = 11.25

t

0.5G

resolution, we observed a small shoulder peak ( < 10%)which might he assigned to the 4:s: or 4157. The sum of io values for 4'. L. The sum of i , values for 4". a

A1 higher

2!4p)O

progression built on

TABLE 111: Ratio of Level.to.Level Emission Intensities from 50 mtorr of D,CO Vs. the Pressure of n-C.H,. PM, torr 0.95 1 3 9 2.84 5.05 6.94

-

[i(4:)/i(4;)la

1.71

1.46 1.49

1.43

1.33 1.32b

i(4:) represents the intensity of peak 2 consisting of two components, 4: and 2p4;. The least-squares fit of the ratio vs. ~ / P M gives the intercept of 1.32. [4']-

112

L 46.

Y

200

oL

[4°1-kd

liil

H2C0

HOC0

Vibrational energy levels (-) as well as lhe heights of the inversion barrier for the double minimum potential in 0'. (n)above llm electronic wlgin of H,CO. HDCO. and D2C0 (i\ 'A") are shown on the len. The Boltzmnn population. N(E,). at rmm temperature is shown as a function of the vibrational energy on the right. Flgure 1.

of n-C4Hto from 0.95 t o 6.94 torr and it reaches an equilibrium value of 1.32 as shown in Table 111. HDCO. The emission intensity distributions of HDCO (A), obtained by 353.0-nm laser excitation of the 4; transition, are shown in Table IV. T h e observed LTL intensities were normalized to the 382.4-nm band (peak 3), since this band can be assigned to the 4: transition, possibly contaminated by the 4; 5: transition t o a small extent, and is more importantly free of contamination from the emission band originating from the 4' levels populated

Figure 2. The examplar first position fluorescence emission spectra which were excited by laser at -353 nm. and recorded by OMA in the 360-460-nm region showing the effectof buffer gas. Top panel shows (a)H,CO (0.15 torr) and (b) H,CO (0.50 torr) with n-C,H,o (40 torr). Middle panel shows (c)HDCO (0.040 torr) and (d) HDCO (0.50 Iwr) with n-C,H,, (5.0 torr). Bottom panel shows (e)D2C0 (0.006torr) and (I) O&O (0.050 torr) with n-C,H,, (5.1 torr). through collisional processes. Unlike the case of D,CO, the problem of peak congestion and overlap for HDCO is somewhat worse than in the case of H,CO. Again, a pure SVL emission spectrum of the 4 O level of HDCO (A) is

942

The Journal of Physical Chemistry, Vol. 83,

No. 8, 1979

Shibuya et al.

TABLE IV: Observed Level-to-Level Fluorescence Emission Intensities from HDCO Excited a t 353.0 nm (Corrected for Spectral Response) 13 0.04

14 0.10

12 0.20

368.7 377.0 382.4 393.6 399.5 408.6 415.2 421.5 428.1 438.6 447.0 453.8 462.6 -468 -473 -481 -490

0.28 0.23 1 0.64 0.58 1.59 0.91 0.39 0.63 1.18 1.06 0.46 0.27

0.32 0.22 1 0.58 0.63 1.29 0.87 0.44 0.69 1.12 1.11 0.65 0.49

0.53 0.23 1 0.96 0.84 1.50 0.93 0.72 0.96 1.43 1.23 0.66 0.66

1

2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 a

S5 6a 10 8 0.46 0.51 1.00 (re1 to the int a t 382.4 nm)

run no. HDCO, torr peak no. kern, nm

i&

1.46 1.07 0.75 1.24 1.27 1.45 0.87 0.74 0.46 0.56 0.75 0.57

Intensities from the second position spectrum, normalized to the i&&

0.62 0.18 1 1.18 1.06 1.53 1.01 0.82 1.14 1.23 1.09 0.58 0.47

0.68 0.23 1 1.40 1.16 1.58 1.00 1.00 1.21 1.34 1.13 0.74 0.67

11 1.51

9 2.00

0.75 0.17 1 1.33 1.19 1.47 0.98 0.92 1.28 1.26 1.08 0.63 0.58

0.82 0.22 1 1.31 1.18 1.38 0.96 0.93 1.34 1.39 1.29 0.82 0.86

values of peaks 6-10 in run no. 8-11.

TABLE V: SVL Fluorescence Emission Intensities and Transition Assignment from 353.0-nm Excitation of HDCO 4’ level peak no. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

A,,

nm

368.7 377.0 382.4 393.6 399.5b 408.6 415.2 421.5 428.1 438.6 447.0 453.8 462.6 -468 -473 -481 -490

ina 0.21 1.00

i

4:

1.8

- 1.2

-0.3 1.48 t 0.11 0.95 i 0.05

4: (415:) 2:4: 2,04: 2:4: 4:, 1;2:4: 1:2:4: 2,04f, 2841 (2:4:53 2:4: (2,O4:5:) 2?4!

1.28 0.11 1.14 5 0.09 _+

(s0.4) (G0.7)

1.3 1.7 0.9 0.9 (G0.7)

2:4q

(G0.7) xi, s 10.2d

2t4:

* Analvzed as overlapping transitions.

TABLE VI: Ratio of Level-to-Level Emission Intensities from 0.50 torr of HDCO Vs. the Pressure of n-C,H,, [i(4i)/i(4:)]

transition

ima

1.00

0.03

a The values in parentheses are from run 556. sum of i, values