Spectroscopy and photophysics of the CF2Ã1B1-X̃1A1 system

JOURNAL OF MOLECULAR SPECTROSCOPY 78,

Spectroscopy

I- 1.5 (1979)

and Photophysics of the CF, A lS,-k

lA, System

DAVID S. KING, PETER K. SCHENCK, AND JOHN C. STEPHENSON Laser Chemistry Program,

National Bureau of Standards,

Washington,

D. C. 20234

Laser excited single vibronic level (SVL) fluorescence and SVL fluorescence excitation spectra of the low pressure vapor phase CFe A-x transition are reported. The spectral origin is at 268.74 nm (37 197 cm-l); extensive progressions in the bending mode v;” = 666 2 5 cm-l dominate the spectra; weaker combination bands involving the symmetric stretch v’i = 1186 r~_15 cm-’ are observed. Fluorescence bands appear that may be assigned as originating either from the 1’2” or the 2”3* vibronic levels, giving vi = 976 _’ 24 cm-* or vi = 900 2 20 cm-*, respectively. Measured vibronic band intensities (Y’ ) v”)* are given for all transitions from the upper states 2” for 0 5 n; I 6. The collisionfree d (O,O,O) state radiative lifetime is 61 2 3 nsec. This same lifetime is observed even for vibronic states containing 8000 cm-’ excess vibrational energy (i.e., the n; = 16 level). SVL fluorescence spectra and radiative lifetimes were used to calculate the transition dipole moment I?, = 1.22 D for this system, and a low resolution absorption cross section 0(2X; 300 K) = 6.7 x lo+’ cm2 for the A-8 origin at band maximum (268.74 nm). I. INTRODUCTION

The dtiuorocarbene radical, CF2, is easily produced by various techniques (I). In the stratosphere, CF, results from the photodissociation of chlorofluorocarbons by sunlight. In laboratory experiments, moderate steady state levels have been achieved in discharges, while high transient densities have been observed following uv flash and pulsed ir photolyses (2). CF, is remarkably stable (3,4) for a radical species, presumably due, in part, to its having a singlet ground electronic state (5). This species is of considerable chemical interest as a precursor or transient intermediate in fluorocarbon reactions (a comprehensive review of CF, chemistry through 1977 is presented in Ref. (4)). The CF, radical has been used in many 12*13C isotope separation schemes, and the laser-produced radical has been used for selective synthetic work since it may be generated in the presence of coreagents of the type HX, X2, or R,C = CR, to yield end products with desirable chemical or isotopic composition (6, 7). Venkateswarlu (8) made the first observations of uv emission attributable to the difluorocarbene radical in 1950. However, it was not until the work of Mathews (I ) in 1967 that the h ‘B,-2 ?4 1 system origin was correctly identified at 268.7 nm. Absorption and emission spectra have yielded the normal bending frequencies v; = 494 cm-l and v!j = 668 cm-‘. Infrared work (9, 10) has identified the two ground state stretching frequencies at v:’ = 1223 cm-* and vjl = 1102 or 1110 cm-‘. Based on very weak uv absorption features, vi was assigned (I) at 1017 cm-l. Under high resolution the rotational subband structure was correlated 1

0022-2852/79/110001-15$02.00/O Copyright 0 1979 by Academic Press, Inc. All rights of reproduction in any form reserved.

2

KING, SCHENCK,

AND STEPHENSON

to a perpendicular type transition, and together with the microwave spectrum ( f t ) led to the determination of the molecular structure and symmetries of the 2 and A states. Both ground and excited state are bent; although the C-F bond length is approximately equal (1.3 A) in both states, the bond angle increases from 104 to 122” upon electronic excitation. Recent work in the matrix has given fluorescence lifetimes (12) for vibrationally relaxed A state CF, ranging from 3 1 nsec (neon) to 23 nsec (krypton). The intense absorption features in the spectral region 220 to 280 nm present the possibility of monitoring fluorocarbon reactions by real time spectroscopic diagnostics. Dalby (3), in early flash photolysis work, studied the kinetics of CF, dimerization by monitoring the strong CF, absorption feature at 245 nm. More recently, King and Stephenson have utilized time and frequency resolved laser excited fluorescence (LEF) to monitor, in situ, the primary photophysics (2, 23) and isotopic selectivities (7) inherent in collision-free ir dissociation of various simple halocarbons. Quantitative spectroscopy of this radical is particularly timely because of the need for remote monitoring of this species in the stratosphere and for in situ determinations of transient CF2 densities formed in the discharges used to excite excimer lasers, since CF, strongly absorbs KrF laser emission at 249 nm and may reduce laser efficiency either by absorption or by collisional quenching of excitation. Because of these applications, we report the zero-pressure radiative lifetimes, transition wavelengths, and normalized spectral intensities for many vibronic transitions within the CF, A-.%? system. II. EXPERIMENTAL

Difluorocarbene

DETAILS’

Production

initial attempts at generating CF, involved the microwave discharge of various CF, containing species. This resulted in low steady state concentrations, e.g., lOlo to 10” molec cm+. The highest practical CF2 yields were obtained by flowing C,F, in He at 1: 1000 dilution through the discharge region at a total pressure of 1 Torr. Higher concentrations resulted in polymer formation in the flow tube and fluorescence cell. Lesser yields were obtained in discharges with nitrogen or argon as the carrier gas or by titrating C,F, into streams of active He or N,. Negligible CF, formation was observed in discharges of CF,, CF2C12, or l,lCzF4C12diluted by He. High transient concentrations of CF2 from various difluoro methanes, ethanes, and ethylenes were achieved using infrared (ir) photolysis pulses from a CO, TEA laser. The details of the collision-free multiphoton dissociation of simple halocarbons by megawatt ir pulses are presented elsewhere (13). The basic approach, however, entails loosely focusing the 0.2 J, 120 nsec FWHM, ir pulses into the center of a large fluorescence cell. The resulting focal zone of highest, nearly uniform intensity is about 2 cm long by 0.1 cm in diameter and is well removed from cell ’ Certain commercial materials and equipment are identified in this paper in order to specify adequately the experimental procedure. In no case does such identification imply recommendation or endorsement by the National Bureau of Standards, nor does it necessarily imply that the material or equipment identified is necessarily the best available for the purpose.

CF,: SPECTROSCOPY

AND PHOTOPHYSICS

3

walls and windows. For peak laser intensities of ca. lOa W/cm2 and wavelengths near 9.3 pm, we observed collision free yields of CF2 as high as 20% for neat samples of a variety of fluorocarbons at pressures in the 1 to 50 mTorr regime. The efficiency of the dissociation process CFzXY + nhv + CF2 shows a steep ir laser intensity dependence. Therefore, all dissociation products are formed in a spatially well-defined volume in the center of the fluorescence cell. Under tlowing conditions wall effects became unimportant and there is no observable polymer formation, even at instantaneous concentrations of CF2 as high as 10” molec cme3. Laser Spectroscopy

and Kinetics

A high power N1 laser (Molectron UV 1000) was used to pump a tunable dye laser of the Hansch design. The oscillator output was amplified and then frequency doubled in angle tuned nonlinear crystals (KDP and KPB). The doubled output was typically 10 to 60 ,uJ in pulses of 3 nsec FWHM duration and 0.5 to 2 cm-’ spectral bandwidth, tunable from 215 to 320 nm. The experimental setup consisted of a large cross-shaped flow cell mounted in front of a fast, moderate resolution monochromator. The uv and ir lasers were aligned colinearly, with their beams propagating in opposite directions, parallel to the entrance slits, and loosely focused in the region of the cell viewed by the monochromator. The sample gases flowed under vacuum at a rate sufficient to flush the cell volume between successive laser shots. Synchronization of the two lasers, such that interrogation of the d state CF, by the uv pulse occurred at some specified time following the dissociation pulse, was controlled to within ?5 nsec with a programmable pulse delay generator (BNC 7030). The laser excited fluorescence (LEF) was dispersed in a f/l .6 uv monochromator with a linear dispersion of 7.0 run/mm and detected with a lP28 photomultiplier tube (PMT). For spectroscopic measurements the PMT output was sampled by a PARC 164/162 gated integrator with digital and ratio options, normalized to uv laser intensity, and digitally stored in a multichannel analyzer for averaging or readout onto either an X-Y recorder or magnetic tape storage. The fluorescence spectra presented herein were recorded at a spectral resolution of 0.3 nm. The position of the fluorescence band maxima, relative to the excitation (laser) wavelength could be determined to an accuracy of &O.15 nm. The linearity of the sine bar drive was checked using a low pressure Hg lamp; the wavelength dependent efficiency of the optical detection system was determined using an NBS calibrated deuterium lamp source. Single vibronic level (SVL) fluorescence excitation spectra were obtained by monitoring a particular v’ + ZI”fluorescence transition (through the f/l .6 uv monochromator) while scanning the uv laser excitation wavelength through the Y’ + u = 0 region of absorption. The spectral resolution of such an excitation spectrum is limited by the bandwidth of the uv laser, in this case about 1.5 cm-‘. The LEF signals were normalized to absolute uv intensity, on a shot-to-shot basis, and recorded as a function of the uv laser wavelength. The fluorescence decay rate measurements were made using a Tektronix R7912 transient digitizer and minicomputer system. The instrumental time response determined with uv laser scatter indicated a maximum fall time (laser pulse + elec-

4

KING, SCHENCK, AND STEPHENSON

A_ III

I

I

I

I

300

I

I

I

350

WAVELENGTH

I

I

I

I

I 400

( nm)

FIG. 1. Single vibronic level fluorescence spectra of A state CF2. A frequency doubled dye laser tuned to 268.74 nm excited the CF, d + 8 origin; tuned to 261.76 nm, the 28 transition. Readily apparent are the long progressions in v’;. The spectral features observed about 150 and 300 cm-l to the blue of the k(O,a,,O) bands are transitions to the x(l,u,-2,0) and x(2,1+4,0) levels, respectively.

tronics) of 2.8 & 0.3 nsec. PMT linearity and gain were calibrated using diffused uv laser scatter and uv neutral density filters. III. RESULTS

Spectroscopy Dye laser excitation of CF, at 268.74 nm resulted in an extensive fluorescence spectrum to longer wavelength (see Fig. l), characterized by sharp, regularly spaced vibronic bands. No emission was detected to the blue of the excitation wavelength; the relative intensities of the bands did not change upon adding inert gases, even at pressures of one atmosphere; and the normalized Franck-Condon envelope exhibited a single intensity maximum, conclusively demonstrating this to be the CF, A ‘II,-_% *A, electronic origin, as indicated by Mathews (I ). The long, prominent progression can be assigned2 as 20, where nI = 0, 1, 2, . . . , 20 z A brief explanation of the vibronic notation for the assignment of spectroscopic transitions and for the designation of specific SVLs follows. The numerical superscript on the right of the ith numbered

CF,: SPECTROSCOPY

AND PHOTOPHYSICS

250 I

300

1

1

1

I

I

II

40,000

I

,

I

350

II

400nm

llII[IIll~Ill

I

I

I,

35,000

I,

,

I,

30,000

I

1,111 25,000

C t-f-I -’

FIG. 2. Histographic representation of the measured normalized spectral intensities of laser-excited SVL fluorescence transitions A(O,o,,O) -+ &uI,uZ,O), for 0 I u; I 6. The quantum number assignments of the lines are given in Table I; spectral profiles in Fig. 1.

and2 = involving the 1: 2: gressions

666 + 5 cm-‘. Weaker transitions are observed to ground state levels the promotion of one or more quanta of the symmetric stretch, u; (i.e., transitions). These bands function as Herzberg-Teller origins for proin vi.

vibration specifies the vibrational quantum number of the upper cal subscript on the right specifies the n; of the lower electronic designation for a mode means a O-O transition for the mode. n; = 0; n; = 2; hj = 1 c, n; = 0; n!: = 0; n; = 1, and may absorption or emission. The term 22 designates the SVL of the For convenience the origin is designated 28.

electronic state (n;) and the numeristate. The absence of the transition For example, 28 3: is the transition represent this transition in either excited electronic state with n; = 2.

KING, SCHENCK,

AND STEPHENSON

TABLE I Wavelengths and Normalized Spectral Intensitie@

n”(v, .vz,v3 Y

x (v, .vz.v31 0.0.0 0.1

.o

o,o,o

2.1

0,3,0

0.4.0

0.5.0

'3.6.0

261.7 ,051

25a.57 ,123

255.00 ,151:

251.89 ,288

248.35 ,287

273.58 ,018

269.89 ,056

266.08 ,152

262.92 .254

259.?c. .136

256.23 ,088

252.69 ,015

262.75 .OlO

260.03 ,014 260.85 .029

256.50 .136

265.19 ,138

260.95 ,065

273.63 ,002 270.57 ,039

283.65 ,074

1.2.0 O,Q,O

0.2.0

265.15 ,018

1 >l .o

0.3.0

.o

System

268.74 .005

1,o.o

0.2.0

0.1

the CF2 A-2

289.32 .113

274.83 ,118

270.96 ,173

267.83 .123

264.15 ,007

278'.51

275.31

271.37 .040

267.45 ,009

280.01 ,133

276.02 .094

272.46

268.60 ,032

233.83 ,012

280.21 ,016

276.44 .022

285.35 ,112

281.11 ,011

277.64 .045

273.30 ,075

280.42

276.00 .OOl

,o

1,3.o

233.63 .008

289.81 .015

285.60 ,010

0.5.0

295.00 .125

290.93 .05a

286.53 .016

2.2,O

278.35 .026

275.24 .015

270.88 .067

280.12 ,047

275.50 ,020

284.20 ,008

279.74 ,018

286.94 .007

282.50 .012

288.37 .037

283.40

300.06 .002

290.61

287.05 ,001

232.68 ,010

295.47 .012

0.6.0

300.96 ,121

296.62 ,013

1.5.0

305.70 ,018

301.21 ,007

0.7.0

307.12 ,111

302.51 ,008

291.86 .057

297.77 .063

293.99

265.18 .013

274.15 .OlO

205.54

299.47 ,017

270.19 ,030

277.30 .005

294.17 ,001

1.4.0

2,3,o

282.82 .079

264.10 ,005

289.25 .035

285.56 .039

295.73

3,2,o

2.4,O

310.54 ,004

1,6,o

312.15 .015

307.24 .002

302.39 ,006

298.95 ,007

Ov8.0

313.56 ,089

308.40 .030

303.62 .036

300.14 ,016

292.40 ,001 289.90 .013 295.10 ,046

290.99 .OOl

299.85 ,020

295.60 .003

301.01 .007

296.96 .020

285.82 ,034

309.98 .002

3.3.0

2,s.o

317.78 .003

1.7.0

318.59 .014

313.22 ,002

308.66 ,012

0,9,0

319.92 .055

314.86

309.58 .006

,050

306.08 ,040

290.97 .OlO

CF,: SPECTROSCOPY TABLE

0.0.0

0.1 ,O

AND PHOTOPHYSICS

I-Continued

o.z,o

0.3.0

317.02 .OOl

0.4.0

324.02 ,003

318.83 ,002

1,8,0

325.51 ,011

319.89 ,006

315.21 .OlO

O,lO,O

327.04 ,034

321.41 ,064

316.57 ,004

2.7.0

330.59 ,002

1.9.0

332.80 .009

326.79 .Oll

O,ll,O

334.69 ,018

328.43 .067

322.95 ,021

319.00 ,011

313.50 ,040

2.8.0

338.31 ,002

331.53 .OOl

327.85 ,004

322.59

317.25 .007

l.lO,O

340.33 .006

334.12 ,011

0.12.0

341.54 .012

335.69 .041

2,9,O

345.96 .002

339.72 ,001

1.11.0

347.78 ,005

341.30 ,012

0,13,0

349.27 .004

343.17 .028

0.6.0

300.76 ,002 305.85 ,024 312.51 .037

307.27 .007

295.86 ,014 303.04 ,021

320.5 ,004 317.47 ,008

307.82 ,008

300.75 ,004

309.45 ,003

303.19 ,020

314.33 ,009

306.99 ,016

320.10 ,042

315.69 ,002

309.16 .021

329.95

323.75

319.22 .002

331.04 ,002

325.40 .009

320.74 .004

314.05 .005

332.56 ,007

326.90 .020

322.21 .OlO

315.68 .005

324.23 ,011 330.02 .036

337.35 .039

336.76 .002

347.91 .003

326.01 ,002

1.12,o

356.03 ,003

349.20 ,007

343.01 .008

338.12

332.40 ,020

0.14.0

357.24 .003

351.01 ,018

344.45 ,034

340.08 ,016

337.00

2,11,0

0.5.0

304.35

2.6.0

2.10,o

7

355.38 ,003

1.13.0

362.46 .OOl

357.31 ,006

0,15,0

363.68 ,001

359.26 ,010

344.11 ,002 350.63 ,009

320.51 ,015 328.99 ,011

322.20 ,005

333.07 .OOl

345.86

339.80 ,015

334.80 .003

327.08 ,006

347.28 .021

341.45 ,004

336.49 ,005

329.26 ,012

342.03 .005

333.60 ,005

2,12,0

363.86 .005

356.84 ,001

351.74

344.5 ,002

1,14,0

365.58 ,004

358.84 ,008

353.66 .004

347.5 .003

0.16.0

367.59 ,005

360.78 ,013

355.56 ,018

350.49 .020

2.13.0

372.19 .ooz

365.68 ,003

1.15.0

374.63 ,001

367.34 ,001

361.82 ,005

355.40 .004

349.63 .004

0.17.0

376.50 ,001

369.52 .OOL

363.78 ,014

358.42 ,027

351.26 ,001

336.59 ,012

352.10 ,004

343.90 .004

KING, SCHENCK,

8

TABLE

0.0.0

0.1 ,o

AND STEPHENSON

I-Continued

0,3,o

0.2.0

0.4.0

0.5,o

360.71 ,001

355.60 ,002

0,6,o

2.14.0

374.12 ,003

1,16.0

376.54 .003

370.60 .003

362.90 ,004

357.50 ,001

0,18,0

378.50 .003

372.56 ,010

367.03 ,027

359.40 ,003

2,15,0

383.43 ,002

377.35 ,001

369.33 ,001

363.75 ,001

1,17,0

385.42 -001

379.53 ,004

371.52 ,013

0.19.0

388.00

381.77 .006

376.80 ,020

367.82 ,005

l,lE,O

395.19 .OOl

388.79 .003

380.94 ,015

374.44 ,001

0,20,0

397.80 .OOl

$91.14 ,003

386.28 ,018

376.78 ,004

2,17,0

402.81

396.15 ,001

388.18 ,004

1.19.0

405.29

398.60 ,002

390.59 .009

383.40 ,002

O,Zl,O

407.96

401.33 .OOl

395.47 ,013

385.74 ,004

l,ZO,O

408.68 .OOl

403.52 .OO?

392.79 ,002

0,22,0

411.68 .OOl

405.64 ,004

395.40 ,003

386.70 .Oll

1 ,21 ,o

414.72 ,004

402.57 ,002

393.33 .006

0,23,0

416.44 ,004

405.56 ,001

396.04

1,22.0

425.35 ,004

413.16 .002

403.37 -007

0,24,0

427.65 ,001

.ooz

349.89 ,010

358.03 .007 359.93 .004

368.35 .OlO

377.58 .Oll

.008

406.36 .007

0,25,0

417.22 .005

0.26.0

428.62 .002

a)

Wavelengths

are given

intensities
in nm and are accurate

( v,,u~,v~>2

are accurate

to ? 0.15 nm. to t.002

The normalized

or lo%, whichever

spectral

is greater.

Irradiation at 261.76 nm excites the band center of the 21,transition. At low pressures vibrational relaxation is much slower than radiative decay and one may obtain the SVL fluorescence spectra of the 2* level, as shown in Fig. 1. About 80% of the spectral intensity is carried in transitions assigned as 22. This single

CF*: SPECTROSCOPY

9

AND PHOTOPHYSICS

TABLE II Deslandres Table of the Bending Vibration of the CF, A -2 System

1

0 0 1 2 3 4 5 6 7 6 9 10 11 12 13 14 15 16 17 la 19 20 21 22 23 24

37200 658 36542 655 35887 643 35244 691 34554 665 33888 671 33217 666 32551 669 31883 634 31249 680 30569 699 29870 599 29271 648 28623 639 27984

504 588 488 458 481 474 486 496 525 502 535 569 510 509 497

37703 662 37041 666 36375 673 35702 668 35034 672 34362 659 33703 656 33047 640 32408 657 31751 647 31104 665 30439 658 29781 649 29132 651 28481 654 27827 631 27196 644 26553

2 497 38200 629 538 37572 677 520 36895 676 516 36219 656 528 35563 673 528 34890 637 550 34253 680 526 33573 647 519 32926 634 542 32292 713 476 31579 624 516 30956 663 512 30292 658 503 29634 611 542 29023

513

27710 655 5.02 27054 642 26413 647 25766 635 25131 626 24505

3 462 38663 640 452 38023 697 43f 37326 634 473 36692 685 444 36007 659 458 35348 415 3%a 663 432 34005 697 382 33308 646 369 32662 672 410 31990 651 383 31339

427 30061 665 373 29396 609 28787 670 407 28117 635 427 27481 646 421 26834 647 420 26186 627 428 25559 649 405 24910 626 24284

4 541 523 520 527 572 568 608 557 569 550 546 550

520

5

6

39204 658 30546 700 37846 627 37219 640 36579 664 35915

484 39688 672 469 39016 691 479 38325 627 479 37698 698 421 37000 679 406 36321

40254 691 547 39562 588 650 38975 665 612 38310 611 699 37699 793 585 36906

3g5 713 34562 685 33877 665 33212 677 32535 647 31889 657 31231 650 30582

413 3tiia 680 447 35009 653 478 34355 691 453 33665 675 454 32989 683 417 32306 63Q 436 31667 641 445 31027 639 30387 677 432 29710

598

366:897

621

34977 619 34358

492 29278 755 407 28523 631 411 27892 654 404 27238 706 345 26532 651 322 25801 601 369 25279 634 362 24645 639 24006 629 23377

569 578 648 653 638 638 644

28461 645 27816 637 27179 646 26533 616 25917 633 25284 633 24650

566

693

667

32973 637 669 32336 668 642 31668 641 640 31028 665 652 30362 661 29701 63I 609 29070

596 607 560 569 593

27775 635 27140 663 26477 624 25853 610 25243 641 24602

progression in Y;’exhibits three intensity maxima at ni = 2,7, and 13. HerzbergTeller origins for progressions in v$ are observed for n; = 1, 2, and 3. From the vibronic intervals observed, we obtain2 = 1186 2 15 cm-l. The progressions in vg originating at all n; # 0 levels exhibit Franck-Condon envelopes qualitatively similar to the An, = 0 progression. For transitions involving changes in n 1, the observed intensities rapidly decrease in magnitude with increasing An,. SVL fluorescence spectra were obtained in similar fashion for the nb = 0 to 6 levels.

10

KING, SCHENCK, AND STEPHENSON

The SVL fluorescence signals were corrected for the wavelength response of the optical detection system and treated in the standard manner (14) to give the normalized spectral intensities, (v’ 1u”)‘: ’

‘I)(VB’_~)*(~~,,U”~~,~~)-l

(v’ 1u”)2 = ~~~~~(a,.~,).(4_IE,.-,)’ a11d

is the observed signal on the vibronic transition u’ + V” ocwhere S v’+~(i)o,_-2~~) curring at frequency &+U,, Eyr+ is the efficiency (PMT volts per emitted watt) of the optical detection system at i)D+,fl, and the summation is over all 0” for a particular v’. A composite listing of wavelengths and normalized intensities of the stronger vibronic transitions from ul = 0, 1, . . . , 6 is presented graphically in Fig. 2 and in Table I; a Deslandres Table of the bending mode transitions, in Table II. Fluorescence excitation spectra were taken to determine, to an absolute wavelength accuracy of 0.05 nm, the band maxima for the 22 transitions with nk = 0, 1, 2, . . . ) 14. An example of one such SVL fluorescence excitation spectrum is shown in Fig. 3. This is a single vibronic band, the 28 transition. The subband structure is readily apparent for ‘R( AK = + 1, AJ = + 1) transitions for values

I 259

I

I

I

260

261

262

I 263 nm

EXCITATION WAVELENGTH FIG. 3. Fluorescence excitation spectrum of a single vibronic band of CF,, the 6(0,2,0) + f(O,O,O) transition. The exciting laser bandwidth was 1.5 cm-r (FWHM). The spike-like features on the short wavelength side of band maximum correspond to the intense R-branch transitions. The particular feature just above 261.0 nm corresponds to transitions from K” = 6. The observed rotational band profile is consistent with a room temperature rotational distribution.

CF$: SPECTROSCOPY AND PHOTOPHYSICS

11

of K > 4. The observed spectra of this type were in good agreement with the rotational data of Mathews, where available. From these spectra we obtain the value Z = 4% -C 5 cm-l. Franck-Condon factors for transitions from the vibrationally relaxed ground state to A state levels involving promotions of ~1 or VAare very small. Previous assignments (1) of vi are tenuous, being based on weak absorption features, and, unfortunately, there is no information available for ~4. We present here prominent CF, fluorescence features involving either A( 1&O) + k( 1J&O), or &O,v;, 1) + _QO,t&l) transitions. Fluorescence spectra observed following SVL laser excitation to the CF, 24 level are presented in Fig. 4. The uv laser was tuned to 255.0 nm and the spectra recorded at 0.3-nm resolution. The two spectra were obtained under distinctly different conditions. The lower trace shows the initial portion (250 to 280 nm) of the resulting fluorescence of CF, dilute in Ar at a total pressure of 2 Torr. At such low Ar pressure there is negligible vibrational relaxation during the 61-nsec lifetime of the A state, and all the fluorescence originates from the initially populated SVL, the 24 level. The upper trace shows, over the same spectral region, the fluorescence spectrum of CF, diluted in Ar at a total pressure of 85 Tot-r following the same SVL laser excitation at 255.0 nm. Five new spectral features, indicated by the arrows, appear that are not assignable to emission from the photo-

I 250

I

I

260

I

I 270

I

I 280 nm

FIG. 4. Fluorescence spectra of CF, dilute in Ar following 2’ SVL laser excitation at 255.0 nm. In the lower trace, the total pressure was very low (2 Torr) so that fluorescence occurred before collisions could cause vibrational energy transfer in the A state. The progression in 6, 2: is indicated. Also readily distinguishable is the lf 2j band. The upper spectrum was acquired at a total pressure of 85 Torr. Five pressure dependent features are marked by the arrows. These features may be assigned as transitions 1: 2: or 2: 3: and arose from collision-induced vibrational energy scrambling in the laser-excited CF,. See text.

12

KING, SCHENCK,

AND STEPHENSON

excited 24 SVL. The intensity in these five features was observed to scale linearly with Ar pressure, consistent with their appearance being due to CF2-Ar collisions redistributing the vibrational energy within the electronically excited species. The five indicated bands havefrequencies consistent with emission originating at the 23 level; they cannot be assigned spectrally to transitions originating from any other CF, 2” level for n; i 6. The intensities of these bands, however, are clearly not consistent with our measured 23 SVL fluorescence spectra. In particular, the five pressure dependent peaks indicated in Fig. 4 have approximate relative intensities 1:5:9:6:4 whereas the 23 SVL spectrum exhibits the relative intensities 4:9:5:0:2 (nl = 0 to 4) at these same wavelengths. The most notable difference is apparent for the fourth peak (d), at 272.5 nm. The 2: transition, expected at 272.46 nm, is missing in the 23 SVL fluorescence spectrum due to its extremely low spectral intensity (i.e., (0301030)2 5 0.001); in the pressure dependent spectrum feature d is strong. We interpret this feature as the CF, 1: 2f or the 22, 3: transition. The fluorescence features observed at 263 and 268 nm [the second (b) and third (c) pressure dependent peaks] result from spectral overlaps of the moderate strength 2: and 1: 22,or 2%3: for peak (b); of 2; and 1: 24 or 27 3: for peak (c). We have observed similar spectral features to appear following other A(O,uz,O) SVL excitations of CF, dilute in high pressures of Ar. As above, these fluorescence bands could be assigned as originating from the CF, A(O,v,- 1,O) state on the basis of transition frequency, but have spectral intensities much too large to be consistent with our measured zero pressure SVL results. If such spectral features are assigned to A(l,v&2,0) + g( 1,&O) transitions, one deduces v;-u; = 210 ‘_ 10 cm-l, or, if v; = 1186 5 15 cm-l, then vi = 976 + 24 cm-‘; if the features are due to &0,+2,1) +~(O,v~,l) transitions, then z&v; = 210 + 10 cm-‘, or, if vl = 1110 cm-l, (10) then v; = 900 cm-l. We note that 1122 and 2231 levels are close in energy to the 24 level, and it is known (16) that collisional processes such as A CF,(0,4,0) are particularly

+ Ar + A CF,(1,2,0)

+ Ar + AE

fast when AE is small.

Lifetime

Zero pressure radiative lifetimes were measured for all CF, 2” vibronic levels with& = 0 to 16, i.e., all readily accessible vibronic states at h > 220 nm. Figure 5 shows typical fluorescence decay data. Here we display both the real time PMT signal and the logarithm of the signal following SVL laser excitation of the 24 level at 5 mTorr total pressure. All SVL lifetime measurements were made under conditions where molecular and wall collisions were unimportant; all observed SVL decays were consistent with the radiative lifetime r,. = 61 f 3 nsec. SVL fluorescence decay rates were obtained for the totally relaxed A(O,O,O) level and the vibrationally excited 2” levels with n; = 0 to 6 in the presence of added gases. In every case Stern-Volmer plots extrapolated to zero pressure lifetimes of 61 +- 3 nsec. A detailed discussion of collisional quenching of the CF, A state is given elsewhere (15). However, we wish to note here that the A

CF,: SPECTROSCOPY

e" e-l e-2 e-3

C

e-4

15

e-5

cn

5

e-6

%

5

z

F

1 c-

1.0

-

k! 0

.8

-

3 LL_

.6

-

.4

-

.2

-

0

13

AND PHOTOPHYSICS

\ C F2 ii (0,4,0)

1 I 0

I 100

I 200

I 300

I 400 ns

FIG. 5. Time resolved CF, 2: fluorescence following SVL laser excitation at very low total pressure (i.e., 5 mTorr). The lower trace shows the real-time fluorescence decay; the upper trace, the logarithm of the fluorescence signal. The indicated straight line fit represents a radiative decay rate of 1.645 x 10’ set-’ (TV= 60.8 nsec).

state of the CF2 radical can experience IO4 gas kinetic collisions with inert gas molecules without being electronically quenched. IV. DISCUSSION

We have not directly measured #By,the quantum yield of fluorescence from the A state of CF,. Generally in polyatomic molecules, predissociation and intersystem crossing, either collision free or collision induced, occur at rates comparable to the radiative rate and $r is less than unity. However, for A state CF, the observations that 7,. is independent of vibrational energy at least to the n;l = 16 level, and that the observed fluorescence decay rate is unaffected by 104collisions

14

KING,SCHENCK,ANDSTEPHENSON

with inert gas collision partners, can only be understood if the rates of these nonradiative processes are extremely small. Therefore, we feel confident in assigning & = 1 for the CF, A state. The data on T, and (v’ 1u”)~ may be used in conjunction with the measured laser excitation (i.e., absorption) profile shown in Fig. 3, to calculate low resolution absorption coefficients for the CF, A c k vibronic transitions. The integrated intensity of a vibronic transition A CF,(V’) t 2 CF,(u) is given by (14)

where N, is the number density in the x CF,(r) level, &,, is the frequency of the vibronic transition, and the electronic transition moment is related to the radiative lifetime by (14) -1 7,

-

z

2

i&v(U’

1v”)2Z?:.

(2)

U”

For the CF, A +x bands, R : = 1.5 x 1O-36 erg cm3. Near the center of any vibronic band, there are greater than 15 individual rovibronic transitions per cm-’ spectral width (I). Our pulsed dye laser had a bandwidth of about 1.5 cm-’ (this is typical of N,-pumped pulsed dye lasers) so that within our spectral resolution were many individual, Doppler broadened transitions. If there are many such individual transitions within an instrumental bandwidth, it is useful to define a low resolution absorption coefficient. Graphical analysis of the band profile shown in Fig. 3 (T = 300 K), gives kmax(cm-l) =

kdvll70 cm-’ i

(3)

where km”” is the absorption coefficient at the band maximum. For the A + 2 origin kmax (cm-‘) = 6.7 x 1O-1Qx N, (cmm3). The strong 28 absorption centered at 249 nm coincides with the emission of the KrF laser, and CF, may interfere with KrF laser action, either as an absorber or by collisionally quenching the excitation. The low resolution absorption coefficient at the center of the 249~nm band is km”” (cm-l) = 4.2 x 10-l’ x N, (cmm3). Absorption coefficients at other wavelengths may be calculated (for T = 300 K) from Eqs. (l)-(3) and the data in Table I. As discussed above, the assignment of the collision-induced fluorescence features to transitions either from the 1’2” or the 2”3l is certain, but there is no justification for choosing one assignment rather than the other. There is also no reason to suppose that collisions would preferentially populate VI rather than vj. Indeed, if V; is populated by collisions, then one could expect vj, which presumably lies at essentially the same energy and has the same degeneracy, to be equally populated. If vj, and u; are populated to a similar extent by collisions, then both states would fluoresce on the &1,&O) + k(l,u,,l,O) or &O&l) + ~(O,v;,l) transitions with comparable intensity. However, there were no unidentifiable spectral features in these high pressure experiments. It is therefore likely that the 1’2” and 2”3l states fluoresce at the same wavelengths. This im-

CF,: SPECTROSCOPY

AND PHOTOPHYSICS

15

plies that vi = 976 + 24 cm-’ and that vs = 900 + 20 cm-‘. Unfortunately these frequencies cannot be positively assigned until the 1’2” or 2”3l levels can be cleanly excited. Optical excitation of such SVLs in a one photon process in the gas phase will be difficult, since transitions of the type 2; 3; are symmetry forbidden (27) [(ng - nj) must be even], ivhile I/, 28 features are overlapped by the intense 2;+2 transitions. In a low temperature matrix, however, the linewidths of the vibronic transitions may be sufficiently narrowed to permit the absolute assignment of vi by laser-excited fluorescence excitation spectroscopy. ACKNOWLEDGMENT This work received partial support from the Department of Energy under Contract No. EA-77-A016010, task no. A-058. RECEIVED:

November

6, 1978 REFERENCES

I. 2. 3. 4.

5. 6. 7. 8. 9. 10. Il. 12. 13. 14. 15. 16. 17.

See, for example, C. W. MATHEWS, Canad. J. Phys. 45,2355-2373 (1%7) and references therein. D. S. KING AND J. C. STEPHENSON, Chem. Phys. Lert. 51,48-52 (1977). F. W. DALBY, .I. Chem. Phys. 41, 2297-2303 (1964). D. S. Y. Hsu, M. E. UMSTEAD, AND M. C. LIN, American Chemical Society Symposium Series, No. 66 “Fluorine Containing Free Radicals, Kinetics, and Dynamics of Reaction” (John W. Root, Ed.), 1978. J. HEICKLEN, N. COHEN, AND D. SAUNDERS, J. Phys. Chem. 69, 1774-1775 (1965). J. J. RITTER, J. Amer. Chem. Sot. 100, 2441-2443 (1978) and references therein. D. S. KING AND J. C. STEPHENSON, .I. Amer. Chem. Sot. 100, 7151-7155 (1978). P. VENKATESWARLU, Phys. Rev. 77, 676-680 (1950). D. E. MILLIGAN, D. E. MANN, M. E. JACOX, AND R. A. MITSCH,J. Chem. Phys. 41, 1199-1203 (1964): D. E. MILLIGANAND M. E. JACOX, .I. Chem. Phys. 48, 2265-2271 (1968). K. C. HERR AND G. C. F?MENTEL, Appl. Opt. 4, 25-30 (1%5); A. S. LEFOHN AND G. C. PIMENTEL,.I. Chem. Phys. 55, 1213-1217 (1971). F. X. POWELL AND D. R. LIDE, JR., J. Chem. Phys. 45, 1067-1068 (1966). V. E. BONDYBEY, J. Mol. Spectrosc. 63, 164-169 (1976). J. C. STEPHENSONAND D. S. KING, J. Chem. Phys. 69, 1485-1492 (1978). G. W. HERZBERG,“Spectra of Diatomic Molecules,” pp. 20,200,383, van Nostrand, Princeton, N. J., 1950. J. C. STEPHENSON,D. L. AKINS, AND D. S. KING, to appear. E. WEITZ AND G. FLYNN, Ann. Rev. Phys. Chem. 25, 275-316 (1974). G. W. HERZBERG,“Electronic Spectra of Polyatomic Molecules,” p. 221, van Nostrand, Princeton, N. J., 1966.