Photodissociation of Co3: Evidence for a long-lived excited state

Volumr 96. number

3

CHEMICAL

Pi-IOTODISSDCI.1TION

Rccc~xcd

29 hwcmbr‘r

OF COT: EVIDENCE

1982; in tin.11 form

PHYSICS

FOR A LONG-LIVED

10 Janua~

S April

LETTERS

1983

EXCITED STATE

1983

l‘nergy .IIIJ~J sk of Ihe phofoir.gment O- Ions produced in the photodlssociation reaction CO, + hv - O- + CO2 idrnttticb t\\o di\tinct O-production mrchanisms. a t\\o-photon absorption via an intermediate bound electronic state, and a LollrGan-Jsstctcd sm$e-photon process via d long-lited excited state. This species has a radiative lifetime exceeding one mizrowsond. 2nd .I collistonJl dissociation cross section measurabiy higher than that of the ground state.

1. Introduction During the p.tst severd ! cdrs we have been engaged studies of the structure md bonding of neg.tttvc tons .uld ion clusters [I]- Tlwsc h.ne included .n~ inwstig.tt~on ofthc photodissociation sprctroscop> of’cc~t.h I.I&~.A~~n~o~~s. RccenIl> _xttcntion has been ~;~LYIX~on COj Jnd its 11)drdtes. species known to ~I.IJ 311 important role in ths chemistry of rhc D regtoII of 111~’iottoq+xe L2.3 ] _Work on the photodisIII dct.Iiled

so~t.tltcw tcxtiott CO; + 111~4 O- + CO, w.ts pronipted b\ J series 0i .trticles [-1-S] in\‘hich mtrty 01‘1hc ti-,nurcs oi‘thc spectroscopy \\ere presented,

includtng

obscrvJtion

of a two-photon

power dependIS]. Nevertheless. there \t’ere mtt~ discrepancies between the finduqs reported. The goal of our work with a crossed once in srbr~.tl olstltc spectrJ1 features

In a recent attempt to probe the dynamical aspects of the photodissociation process, we have added 3 retarding field energy analyzer to the ion optical systern of the spectrometer. This addition has ensbled d deterniin3tion of the kinetic energy of the parent and dsughter ions. and the selective study of certain phorodissociation

pathways.

Initial measurements

of

the ener_gy distribution of tlte photofragtnent O- ions indicate that at least two photodissociation reaction pathways are open to CO,. In addition to the two-

photon

process mentioned

above, a second pathway

involves photon excitation and 3 resulting long-lived state that 113s 311 enhancmment collisional dissociation cross section in comparison to the ground st3te. and a rddkttive lifetime in the microsecond range. Herein. we present and discuss the experimental results which lead to these conclusions_

ion hr.nn-laser Iw3ni pliorc~dissoci~tioxi spectrometer 11~s heen to Jugnient k~iowledge of the electronic

st.nes mvohed

in the pl:~)todissociatiotl

processes of

2. Experimental

1111scarnpl~x system 191. and our data are consistent

with .I ~nod~l III which photodissociation proceeds by absorption of t\c o photons to a purely repulsive state 114.

Ground-state CO, ions are formed in 3 drift cell (at z:j Torr of 3 5% N20/9S% CO, mixture) by three-body association of O- with CO, [ 1 l-141; Ois produced by dissociative attachment of an electron

+ Prehminary stud) of the CO; system commenced zt zhr Unircrslt? of Colorado. ** ke+rnt IdJress: Chcmarry Dspsrrmsnt, Middlebury College. Uiddlebury. Vermonr 05753, USA.

to NzO. After many collisions. the ions effuse into vacuum through a 75 urn orifice, whereupon they are collimated. mass selected by a Wien velocity filter [ 15,161 and focused into the intracavity flux of an argon ion pumped tunable dye laser. Following ener-

~i,r ,tn interttiedi.tte

32s

bound excited electronic

st3te

0 009-2614/83/0000-0000/S

03.00 0 1983 North-Holland

Volume 96, number 3

CHEMICAL

PHYSICS

8 April 19S3

LETTERS

iment of monito~n~ ion intensity as a function of the

i”

_L5

i ti

ICI Ia BErAM ID

3, Rest&s

--

----If

I

retarding potential placed on the central screen of region IV- The actual energy distribution is obtained by differentiating the integral intensity curves. In what follows, we refer to peaks in the enerDT distribution even though they appear as steps in the colkcted data.

II

Ifi

IV

V

Fig. I _ Details of interaction region and energy analyzer. Region I: deceleration lens and remainder of ion optical system. Region 11: field-free interaction region. Region III: quadrupole entrance optics. Region IV: three-screen electrostatic energy analyzer_ Retion Vr quadrupoIe mass spectrometer.

gy analysis (discussed in more detail below). the ionic photofrao,llent and parent ion intensitiesare measured b_v a quadrupole mass spectrometerLaser-induced effects are clearly differentiated from other by chopping the laser and n?onitoring a signal and background channel. Additional details of the experimental appa-

The pressure and field conditions of the ion source were adjusted to give sufficiently thermalized CO? ions to reproduce the wefl-established spectroscopic structure of the ground-state ion Es]. The laser was then set to 575 mii. the center of a large feature in the photodissociation spectrum_ Fig_ Z shows a typicall result of monitoring the intensity of both the parent CO,

ions and the photo-

fragment O‘- ions as a function of the retarding Geld in the energy analyzer. The CO, parent beam has a narrow energy distributiotl centered on 7-25 eV_ In contrast. the O- photofragment energy distribution is bimod& ES indicated by the distinct double step in the retarding field curve. The higher-cner~

peak in

ratus are reported elsewhere [ 171. Since the geometric and electrostatic details of the inreraction region and ensroy analyzer are important for the discussion of the results, this portion of the apparlitusisshown in detail in fig. l_ The final element of the deceleration optics. LS, and the screen around the ~nt~ractiot~ region, IRS. are nortnall_y held at the same potential of l-10 V_ The first element of the quadrupole entrance optics, L6. is used in conjunction with L5 and IRS to control the ion kinetic energy at the interaction region (region II in frg. l)_ I_6 can either be held at the same potential as the remainder of the interaction region. or it can be made slightly attractive to provide a focusing effect_ U3, L9 and the two outer screens of the energy analyzer are usually at 5 V, and L7 is adjusted to provide focusing of the photofragments. It should be noted that any ion whose kinetic energy is greater than or equal to a given value of the retarding potential will be transmitted by the energy analyzer and detected. Therefore, an integral of the ion energy distribution is obtained in tlte usual exper-

Fie. 2. Energy analysis of CO; parent and O- photofragment ions. In the upper graph, total CO; intensity is plotted versus the retardin$ potential on the cenrer screen of region IV; the difference between the htser on and laser off O- intensities is

presented in the lower graph. The retarding potentials of the two O- intensity steps are related by eq. (2X

329

Voh~nc 96. number 3 the distribution distribution.

is exact11 coincident with the CO, whereas the other is seen to be at lower

energy. E.xperiments verified that peak positions were not influenced by focusing conditions of the ion k,ptlcs. .A uurnber IIIWI~IS

of intensity versus retarding field esperconducted in which the kinetic energy of

mere

point of interaction with the laser well-defined values between 1 and 10 eV. In Al such esperimcnts. the position and shape ,,I‘tiw onset of pArent CO? remained unchanged. In .&lition. the position of the high-energy peak in the 0 dlstributton rem.Gned cotncident with the CO, lx~k_ Ilowc\er. the lower energy pe.tk in the O- disIr~i~t~t~w~ trxkcd the interaction region ion energy, the mi

!x~rn

NJS set

.tt

dt

8 April 19S3

CHEMIC.4L PHYSICS LETTERS

the

sc\*rrsl

0 I

appearing at an ever higher retardins field value as the beam ener,q was increased. A competing source of O- in these experiments is the pressure-dependent collisional dissociation of CO, [ 181. It is possible to differentiate purely laserinduced dissociation mechanisms from those that depend on collisional interactions as well, by monitoring the intensity ofcollisionally produced O- in phase with the laser chopper. Fig. 3 shows the result of varying the background pressure on the intensiries of the two peaks in the energy distribution of laser prodttced O-. The laser-off collisionally induced O- signal, which also shows a linear dependence on pressure. has been subtracted_ The bottom data set in fig. 3. taken with the retarding field at 0 V to reject the lowerenqy peak. shows that the higher-energy photofragment peak has a marked pressure dependence_ Conversely. the data set at the top of fig. 3, taken with the retardmg field at 5 V. shows no pressure dependence: this condition allowed all photofragments to be detected_ Obviously. even though a superposition of the effects of both peaks is being measured. the top curve is largely characteristic of the more intense lowerenergy peak. The data were taken under the same conditions as for the experiment in fig. 2. and the pressure WJS changed by throttling the gate valve over the m.titi vdcuutn pump.

4. Discussion The immediate

0

OO

I 4

?R&S”RE

?TorI- x I&)

conclusion

from these experiments are being produced by two distinctly different mechanisms. A detailed consideration of the kinematics of the dissociation process was made which led to information on the spatial distribution of the photofragments and on the partitioning of energy between the various modes of motion of the photofragments. We restrict this discussion to a brief analysis of several features of the kinematics which is sufficient to show that one of the me& anisms must be occurring on a time scale much slower than the other. Consider an ion of mass M travelling with beam energy Eb into the field free region II held at a drift potential ud. The ion absorbs an amount of ener_q from the laser and dissociates into two fragments of masses NIP and ~1~. where ml + m2 = ATI_The energy

is that O- photofragments

Volume 96, number 3

CHEMICAL PHYSICS LETTERS

exceeding the dissociation energy of the ion is partitioned into recoil and internal energy of the two dissociation fragments, where Ee is taken to be their recoil ener,y. In the simplest case, the fragment of mass 211I _ which we take to be ionic and therefore detectable. is ejected in the forward direction along the beam axis. Allowing for internal energy in the products, the maximum energy, E; . of this fragment is given by E;

= (33z~~~~E~~

(33l~~~~E=~~

+ 2(Eb331,fEe332z)f4. (1)

The first term accounts for the fact that before dissociation. the fragment, as a part of the larger ion, was travelling with velocity u= (2EblM)LfZ. The second is the amount of the excess energy E,,partitioned into the fragment. consistent with energy and momentum conservation. The potential of the drift region, tr,. does not appear directly in the expression for fragment energy, eq_ (1). Since the ionic fragment is born at U,, howvever. the potential U, that must be placed on a retarding field after the drift region in order to stop the

fragment must be referenced given by

to V,. This potential

Ux=Ud-El.

where the minus sign is appropriate

is

(9

for the specific cae of a negative fragment ion. and El represents E; divided by charge. Eq_ (2) shows that a change in the drift region potential will directly change the value of the retarding potential required to stop fragments which are born in the drift region (II). The experimental evidence shows that, of the two peaks in the O- photofragment energy distribution, only the lower-energy peak responds in this way to changes in the interaction region potential_ The observation that the intensity of this peak also shows no dependence on the pressure of background gas in the apparatus is consistent with this lower-enerw O- being produced within the laser cavity by a two-photon absorption f&-10] which gives rise to the accepted structure in the photodisso-. ciation spectrum. Further evidence supporting this two-photon process was obtained in recent studies 1171 of the photodissociation of CO, and its hydrates where similar initial transitions were shown to be involved. In this paper attention is focused on the discovery

8 April 1983

that the peak of the higher-energy O- species does not change position in response to changes in the interaction region potential. Under all conditions it has been found to remain exactly coincident with the parent CO, peak_ Eqs. (1) and (2) admit the possibility of this coincidence at a particular birth potentialComplete independence of position on interaction region potential is, however, inconsistent with dissociation of the parent prior to passage through the enerw analyzer. The only remaining possibility is dissocia-

tion of CO, is the region beyond the energy analyzer (regions IV and V). 0”” fragments produced between the energy analyzer and the quadrupole mass spectrometer. though in principle still subject to the considerations of eqs. (1) and (3). would respond to the energy analysis in exactly the Same way as the parent CO, ions. This behavior is exactly what is observed for the higher-energy O- peak. The O- produced by collisional dissociation between the laser and the energy analyzer could provide a small contribution to the lower-energy 0 - intensity, but this contribution would have a different pressure dependence than what is observed. We conclude that a fraction of the CO? parent ions entering the laser cavity undergo a laser-induced series of transitions which leave the ions in an excited state with a sufficiently long radiative lifetime to survive the several microseconds of beam transit time from the laser througil the energy analyzer_ The pressure dependence of the intensity of the O- ions produced by this mechanism indicates that collisional interaction of these excited CO, species With background moleculesis able to provide the additional energy requiredfordissociation. Observation that laserescitationof the CO, beam increases the total amount of collisional O- produced leads to the conclusion that the collisional cross section of the long-lived state is substsntially higher than that of the ground state. Within the limits of error of the present measurements, the intercept of the bottom curve in fig_ 3 is not discernably different fromzero.Therefore, the O- photofragll~ent ions that are detected at the same apparent translational energy as parent CO, must be produced by collisionally induced dissociation of excited COj following transmission of those ions through the energy analyzer_ The exact identity of the long-lived state and the mechanism that populates it has not yet been deter331

Volumr

96, number

CHEMICAL

3

PHYSICS

It is grnerally accepted that there are IWO ne,uly degenerate excited states of CO, [S,19]. We IdenGfy one ofthese.accessible from the ground state by a weaN_\’ allowed optical tranation. as the inter-

nw~ed.

n~edi~re in the two-photon dissociation process [9,10, 171. The other. whose coupling with the ground state is op~~c~ll~ forbidden [5.17]. may become populated hy .I radi~tionlcss rransilion from the allowed state and Ihereby lead IO rhe longlived excited species reported 111 rhs Jxqxr. Nevertheless.

hhry slur

ions

we

canno totally dwount

cxc~ted

the possi-

10 the

intermediate state Ilu~~rescr tusk IO Hugh-lyq vibrational levels of the to the obz~~r~wld butte. These nught then contribute WILY~ mrcm of the collisionJ frJgnientation cross W~~IIWS llo~ever, rapid rcdistriburion of vibrational ~rwrg~ in1~3 various modes of the ion would reduce IIW \d~l.~~~on.d energy in the dissociation coordinate. ~cndc~in~ rhis .iitcrnativc less likely. Additional expe~IIIIC’II~S.Irc in prcqas to probe the derails of rhe >irucItltL’ .intl d\ rlmlics of the long-lived state.

8 April

LETTERS

References 111A.\V. Castleman L.C. Christophorou p- 189; A.W. CastlemanJr..

\xould

Ike IO thmk

I)r.

lztok

Cadez

I’iofc5sor li .-1 krnlieini for helpful discussions. Suppo~ 1 of W US &Iny under Grdnt No. DAAC29s?-K-O 160, the .L\rmospheric Sciences Section of the XIIIOILI~ S&xc Foundakm under Grarir Xo. ATMS2 I 20-S. .UUI tlw Depxlnient of Energy under Grant No DL~-ACO2-7SE~O4776 is gratefully acknowledged. One oi‘us(i\lli) \\ouIJ like 10 rhank the SWISS National Sswrxc Foundation ior p.xlml suppo~r during the ioursc 0i r111swork.

P.M. Holland

and R-G. Keesee,

ed. 1981)

Radist.

R.G. Keesee.

N. Lee and A-W. Castleman

Jr.. J. Chem.

Phys. 73 (1980) 2195. J.R. Peterson. J. Geophys. Res. 81 (1976) 1433. D. Smith and N.G. Adams, in: Gas phase ion chemistry. ed. h1.T. Boaers (Academic Press, New York, 1979) p, 1. R.A. Beyer and J.A. Vanderhoff, J. Chem. Phys. 65 (1976) 2313. J-T. Moseley. P-C. Cosby and J.R. Peterson, J. Chem. Phys. 65 (1976) 2512. M.L. Vestal and G.H. blauclaire, J. Chem. Phys. 67 (1977) 3758. G.P. Smith. L.C. Lee and J.T. Moseley. J. Chem. Phys. 71 (1979) 4034. J.F. Hiller and XL. Vestal. J. Chem. Phys. 72 (1960) 4713. A.\V. Caar1rm.m

Jr.. P.M. Holland, D.E. Hunton, R.G. Keesee. T.G. Lindem.m. K;.l. Peterson, F.J. Schelting and B.L. Upschulre, Ber. Bunserges Ph>slk. Chem. 86 (1982) 866. D.M. Lindsay,

The .tutho~s

Jr.. in: Electron and ion warms. (Pergamon Press, New York,

Phys. Chem. 20 (1982) 57:

1101 A.W_ Castleman

.iiitl

19s;

Jr., D.E. liunton. T.G. Inrcrn. J. Mass Specrrom.

Lindeman

and

Ion Phys.

to be published. Ill1 J.L. Moruzzi and A.V. Phelps, J. Chem. Phys. 45 (1966) 4617. 11’1 D.K. B&me. D.B. Dunkin. F.C. Fehsenfeld and E.E. Ferguson. J. Chem. Phys. 5 1 (1969) 863. iljl D.A. Pxkes. J. Chem. Sot. Faraday Trrans. 168 (1972) 627.

1141 H.W. Ellis, R.Y. Pai. 1-R. Catland. E-W_ McDaniel. R. Wernlund and M.J. Cohen.

J. Chem. Phys. 64 (1976) 393.5. 1151 L. Wahiin, Nucl. Insrr. Methods 27 (1964) 55. 1161 W. \Cien, Ann. Physik 65 (1898) 440: 8 (1902) 260. M. Hofmann. T.G. Lindeman and A.W. 1171 D-E. Hunton, Castleman Jr., in preparation. Planet. Spxe Sci. 29 LlSl R.L C. Wu and T.O. Tiernan. (1981) 735. 1191 J-F. Olsen and L. BumeIle. J. Am. Chem. Sot. 92 (1970)

3569.