Fluorescence spectra of matrix-isolated Se2

JOURNAL

OF MOLECULAR

SPECTROSCOPY

Fluorescence FAKHRUDDIN Department

83, 64-69 (1980)

Spectra AHMED

of Matrix-Isolated AND EUGENE

Se,

R. NIXON

of Chemistry, and Laboratory for Research on the Structure of Matter, University of Pennsylvania, Philadelphia, Pennsylvania 19104

Laser induced fluorescence spectra are reported for samples of natural selenium and of the separated ‘%e and ‘%e isotopes in Ar and Kr matrices. The B(O:) + X(0,+) and B(1,) + X(1,) systems of Se,, already known in the gas, are observed by both single photon and biphotonic excitation considerably red-shifted in the matrices. The A(O,+) + X(0,+) emission of Se,, not observed in the vapor, appears in the matrices with its origin near 15 100 cm-‘. Another system with Q,, = 24 429 cm-’ and 0: = 538 cm-t is thought to belong most probably to some polyatomic Se, molecule. I. INTRODUCTION

The spectrum of gaseous Se, has been repeatedly examined both in emission and absorption. With isotopically enriched selenium samples Barrow et al. (l-2) performed rotational analyses of the band system in the visible and near ultraviolet regions and identified it as the B-X transition separated into two subsystems &0,+)-X(0,+) and &1,)-X( 1,) as a result of the spin-orbit coupling splitting of the Hund’s case b states X(3Cg) and B(3Z;) into the case c states X(0,+), X(1,), B(O:), and B(1,). The cross transitions B( 1,) --, X(0,+) and B(0:) -+ X(1,) were later studied by laser induced fluorescence (3-4). Constants for the four X and B states of gaseous 8oSez are listed in Table I. Absorption measurements on gaseous Se, have also revealed at least five higher energy states in the 51 50055 000 cm-l range (5). These states, designated as C(l,), C(O,+), D(l,), E(O,+), and F(l,), are incompletely characterized. In all of the vapor work, transitions involving the A(0:)have never been observed. The origin of the A(O$)-X(0,+) band system in gaseous Se, would be expected at a little higher frequency than the 19 400-cm-’ origin for the heavier Te, molecule (6). We report here for the first time spectra of Se, in argon and krypton matrices. Matrices were produced by the simultaneous deposition of selenium vapor and the inert gas onto a cold (15 K) copper surface. The temperature (170°C) of the furnace generating the selenium vapor and the inert gas flow rate were such as to deposit about 1 pmole selenium and 15 mmole matrix gas per hour. Both natural selenium and separated 78Se and 80Se isotopes (obtained in 96% isotopic purity from AERE, Harwell, U. K.) were used. Visible and ultraviolet lines from a Spectraphysics Ar+ laser served for excitation and the fluorescence spectra were recorded on a Spex Ramalog system.

0022-2852/801090064-06$02.00/O Copyright 0 1980 by Academic Press, Inc. All rights of reproduction in any fom reserved.

64

FLUORESCENCE

65

OF Se, IN MATRICES

TABLE I Molecular Constants (cm-*) for Se, 80

X(02

SO

Se2 gas*

Se2 in

0

385.30

0.964

0

376.86

X(lg)

510.0

387. 2

0.964

A

379.0

A@;)

-

15131

-

-

B(OJ

25910.9

246.29

l.Olb

22636

WIU)

25985.4

246.42

1.225

(22118

>a Ref.

1-2

A’s

are

+ A)

5

t \

-

0

375. 32

1.00

0.97

h’

376.

1.01

22420

”

(21696+

X0;

“Se2

ii 4300

-~ c w ’ z

/Ar

3 2 W-19436cm

A’)

62

-

L

1 -1

0

0

0

t

in Kr

undertermined.

AO;

z

Se2

0.99

-

1.5

F

80

Ar

f

1

!7300

,

WAVENUMBER

Ccm“)

&L I9000

252 lo

FIG. I. Fluorescence spectra In each case exciting laser line refer to bands belonging to indicates plasma line. Bottom

of isotopic selenium mat~x-isoIated in argon at 10 K at M/A = 6 x 1OW. and its power are indicated. In center and bottom panels, symbols 0 and 1 B(O$) -) X(0,+) and B(1,) -X(1,), respectively. Asterisk at bottom panel also shows seven peaks belonging to system discussed in text.

66

AHMED AND NIXON

“Se, / 15

Kr

I W - 19436

100 mW-

17500

cm-’

0.

1 I

UV

I9000

_ WAVENUMBER (cm-’

I

I

FIG. 2. Portions of the B + X emission system for isotopic and natural samples of Se, in argon and krypton matrices at 10 K and M/A = 6 x 105. Laser line and its power used for excitation as indicated. Top panel shows biphotonic excitation and 0 and 1 refer to B(O,f) + X(0,+) and B(1,) -+ B(1,) subsystems. UV refers to unresolved uv lines (27 488, 28 458,28 482, and 29 976 cm-‘) of Ar+ laser.

II. RESULTS AND DISCUSSION

The strongest feature, extending from 22 700 to 12 500 cm-‘, in the fluorescence spectrum is the B +-X system. A part of that system as excited by uv radiation is shown in Fig. 1 for 78Se2 in argon, where it is clear that the B(O$)--, X(0,+) transitions are more intense than the corresponding vibronic transitions in the B(1,) + X( 1,) system. We put v,,,, for the 0: + O,+subsystem of *OSez in argon at 22 636 cm-l and voofor the 1, + 1, system at 22 119 cm-‘. Our data do not give the magnitude of the energy gap between X(0,+) and X(1,) to compare with the 510-cm-’ value for the vapor but they do show that in argon the B(0:) state is red-shifted by 3275 cm-’ with respect to the ground state. The B(O,+) + X(0,+) transitions appear clearly in matrices prepared with natural selenium. Portions of the system for Se, in argon and in krypton shown in Fig. 2 reveal the resolution of isotopic bands. The values for w, and w,x, given in Table I for the X(0$) and X( 1,) states of 8oSe, in Ar and in Kr reproduce all of the observed B --, X frequencies to within + 1 cm-’ (Tables II and III). The isotopic relations can be applied to the w, and w,x, of 8oSez to calculate vibrational constants for “Se,, ‘?SesoSe, ‘?SeeoSe, and 8oSe*2Seand these reproduce by & 1 cm-’ the observed vibronic frequencies of each of the isotopic molecules. The w, values for the X

FLUORESCENCE

67

OF Se, IN MATRICES

states turn out to be 2.5% smaller for the matrix-isolated Se, than for the gaseous molecule. One notes from Tables II and III that our voovalues for both the B(O:) -+ X(0,+) and the B( 1,) -+ X( 1,) transitions in argon matrices increase regularly by 6- 10 cm-’ from one molecule to the next in the isotopic Se, series: 78-78, 78-80, 80-80, and 80-82. The same trend occurs in krypton matrices. Differences in zero-point energies in the X and B states contribute less than 0.5 cm-’ to these increases and so the values of T,for B *X increase with increasing molecular mass at a rate of 3-4 cm-’ per a.m.u. Interestingly we observed essentially the same variation in the values of T,for the A -+X system of lZ8Tep and 130Te2(7) and for the A + X transition with the isotopic molecules 58Niz, 5sNi60Ni, and 60Ni, (8). The Ni, fluorescence is not known for the vapor but with Se, and Te, the band systems under discussion suffer large red shifts (3000-4000 cm-‘) from the gas to the matrix. It is probably not surprising that small isotopic differences in mass and vibrational frequency alter interactions with the matrix environment sufficiently to produce measurable differences in the molecular potential energy functions. Figure 2 illustrates that the B - X systems can be excited biphotonically with moderate power in the green laser line at 19 436 cm-*. A(G) + X(0;) Each of the visible laser lines at powers in excess of a watt excite a very weak band system with origin near 15 200 cm-’ (Fig. 1). We assign this as the A(0:)+ TABLE Peak Frequencies Argon 80% vll -

0 1 2 3 4

5 6 7 a 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

(cm-‘)

II

of B(O:) + X(0,+)0’ = 0 +

u” Bands

matrices 78Se

2

Krypton 80

2

obs.

CZJC.

obs.

caic.

22636 262

22636 261

22619 240

22619 240

21889 517 148 20781 416 053 19692 334 18977 622 269 17918 568 220 16876 532 191 15852 515 181 14848 517 188

21888 517 148 20781 416 053 19693 334 18977 622 269 17918 569 22i 16876 532 192 15853 515 ial 14848 517 188

21862 487 114 20743 373 006 19640 277 18916 556 199 17844 491 140 16791 444 100 15757 416 078 14743 408 075

21862 487 I13 20742 373 005 19640 277 18916 557 coo 17845 492 141 16792 445 101 15758 417 079 14742 408 075

of Se, in Matrices

abs.

Se

78

Se

ca1c

80

SC 82%

ohs.

21137. 20762

395 030 19667 305 18946

395 830 19667 305 18946

589 234 17880 529 ia0 16833 488 145 15804 465

589 234 17881 529 180 16833 488 144 15803 464

78 Se2

SC2 obs.

22642

22629

21131 20762

cdc.

matriccs

80 talc.

ohs.

talc

21280

21281

20909 539 170 19805 441 078 18719 361 006 17653 301 16952 604 258 15915 574 235

20909 538 170 19805 441

22420

21306 20940 574 211

20436 075 19717 360 005 18651 300 17951 604

20436 076 19717 360 005 18652 301 17952 605

19849 490 132 18778 424 073 17723 375

259 16916 574 234 15898 362

259 16916 575 235 15898 563

029 16686 342 004 15667 333

21306 20938 573 210 19848 489 132 18776 423 072 17722 375 030 16686 345 005 668 333

079 18719 361 m5 17652 300 lb951 603 258 15914 573 233

68

AHMED AND NIXON TABLE III Peak Frequencies VU)

-

(cm-‘) of u’ = 0 + u” Bands of Fluorescence

80 % 0bs.

BllU)

x(1s): *= 78se 7.

ca1c.

ohs.

talc.

8Ose

ohs.

-Xllg):

Kr

7%

2

cdc.

Ob8.

A(O)-

22119

talc.

5 6

14357

19839

3

012

012

13981

13982

373

372

18041

18041

11 12

078 17721

078

011 17651

010 17651

17687

17687

13

368 016

293

16784

15

16667

16667

16

319 15973

319 15973

17 18 19 20 21 22 23 24 25 26 27

629

288 14948 609 272 13938

629 287 14947 608 272 13938

15114

756

19839 470

18736

016

14755

cak.

14357

18737

14

1

ohs.

15131 383

18795 435 17722 368

talc.

15131 303

18794 434

ohs

0 2

9 10

156

Y”

78se 2

20210

19519 155

8

Ar

20210 19520

7

21680

21696

22099

X(O):

*OS2

*

0

Systems of Se, in Matrices

101

294 16938

4

469 101

16938

18702

334

636 290 15946

584

290

233 15883

233

15946 605

535 191 14846

191 14847

505

506 166

lb5 13830 493 162 12829 500

13829 494 161 12829 500

16984

637

585

15883 536

335

265 14926 589 2.54 13924 594

604 264 14926 589 2.55 13923 593

339 17977 618 262 16907 556 206 15858 511 167 14826

18702 339 17978

13642

5 6

276 12911

13643 276

608 236

12911

7

548

548

619

8

263

9

188 11828

188 11829

608 237 12868

12498

500

16908 556 205 15857 511 167

485

14825 485

“11

ohs.

ca1c.

147 13811

147 13811

0 1

24429 23894

24429 23895

2 3

366 22837

364 22838

4 5 6

316 21796

316 21797

283

283

X(0,+) transition, unobserved so far in the vapor. The system has much the same appearance as the A +-X system of Tez in argon (7) except that the zero-phonon lines in the latter system are relatively more intense. Table III shows our choice of 15 131 cm-’ as voOfor s”Se,, representing a red-shift of probably 4000 cm-’ from the gas. 21 000-24 500 cm-’ emission. On the high frequency end of the B -+X system of ?Sez in argon we observe a series of seven sharp bands with spacings of 514-534 cm-’ (Fig. 1). Our assignments of the band frequencies (Table III) are fitted by the parameters: voo = 24 429, wg = 538.53, and (we+)” = 2.03 cm-l. Unfortunately our small supply of 80Se was exhausted before we could examine this system with the heavier isotope. Several possibilities are suggested for the origin of this system: the terminal state is a low-lying excited state of Se,; the emission belongs to an impurity molecule, the most probable one being SeS; or a polyatomic species Se, is responsible. Insofar as a low-lying state of Se, might be involved, an upper limit on the position of that state above the ground state can be set at about 4000 cm-‘, since the band system origin lies at 24 429 cm-l and the fluorescence can be excited by the uv laser lines at 28 458 and 28 482 cm-l. An excited state of such low energy would presumably have to arise from the same . . * (T~)“(T*,)” electron configuration as the ground state “C;-X(0,+) but it is not reasonable that either possibility, ‘A,(2,) or ‘Zi( l,), would have a vibrational frequency 40% larger than that of the ground state. Absorption spectra of SeS (9) yield the following constants for the ground state of gaseous 78Se32S:w, = 555.56, w,x, = 1.848 cm-’ and give To values of 27 216.46

FLUORESCENCE

OF Se, IN MATRICES

69

and 28 138.46 cm-l for the excited A(O+)and B(O+) states. A red-shift of 30004000 cm-’ in the origin of the A + X or B +-X system for SeS in the matrix is quite possible and in view of the present results with Se,, a decrease in the o, value of SeS by 3% from vapor to matrix does not seem beyond question. The peak intensities of the bands in the fluorescence system in question are 250 times greater than those in the A + X system of Se,, suggesting that if SeS were present, its concentration might be more than that of a trace impurity. On the basis of the characteristics of the fluorescence spectrum alone, SeS cannot definitely be ruled out. It should be noted, however, that in matrix studies with tellurium (7) we have observed fluorescence band systems with similarly large vibrational spacings and that isotopic data in that case permit exclusion of the TeS and TeSe molecules as the emitters. Analysis of the vapor above elemental selenium at 473 K by mass-spectrometric methods (10) gives the partial pressures of the species Se,, Se,, Se,, Se,, Se,, Se,, and Se, as about 4 x IO--*, 4 x lo-“, 1 x lo-lo, 3 x 10-7, 1 x 10-6, 4 x lo-‘, 4 x 10P8, and 2 x lop6 atm, respectively. There are thus many candidates for the emission in this 2 1 000-24 500 cm-’ region and any one of them might well have a vibrational mode (presumably a totally symmetric one) with frequency of 540 cm-’ which would appear as a progression in this electronic transition. ACKNOWLEDGMENT This work was supported by the National Science Foundation under Grant No. CHE 76-16724. RECEIVED:

July 12, 1979 REFERENCES

I. R. F. BARROW, G. G. CHANDLER, AND C. B. MEYER, Phil. Trans. Roy. Sot. London A 260, 395-4.56 (1%6). 2. K. K. YEE AND R. F. BARROW, .I. Chem. Sot. Faraday Trans. 68, 1181-1188 (1972). 3. G. GOUEDARD AND J. C. LEHMANN,.~. Phys. B 9, 2113-2121 (1976). 4. D. I. GREENWOOD AND R. F. BARROW, J. Phys. B 9, 2123-2126 (1976). 5. R. F. BARROW, W. G. BURTON, AND J. H. CALLOMON, Trans. FaradaySot. 66,2685-2693 (1970). 6. R. F. BARROW AND R. P. DUPARCQ, f’roc. Roy. Sot. Ser. A 327,279-287 (1972); T. J. STONE AND R. F. BARROW, Canad. J. Phys. 53, 1976-1981 (1975). 7. F. AHMED AND E. R. NIXON. J. Mol. Spectrosc., in press. 8. F. AHMED AND E. R. NIXON, J. Chem. Phys., in press. 9. F. AHMED AND R. F. BARROW, J. Phys. B 7, 2256-2263 (1974). IO. H. KELLER, H. RICKERT, D. DETRY, J. DROWART, AND P. GOLDFINGER, Z. Phys. Chem. 75, 273-286 (1971).