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Fluorescence quantum yields of isotopic CO2+ ions
ChemicalPhysics 33 (1978) 113-121 0 North-Holland Publishing Company
FLUORESCENCE QUANTUM YIELDS OF ISOTOPIC CO; IONS Sydney LEACH, Michel DEVORET* Lnboratoire de Photophysique 91405 - Orsay. France
Molhlaire
du C.N.R.S., Britiment 213, Universit6 de Pariis-Sud,
and John
H.D. ELAND
Argonne National Laboratory, Argonne, 11&z&s 60439, USA Received 24 February
1978
Fiuorescence quantum yields of the x ‘II~ and 6 ‘$ states of the isotopic ions 1*C’60;, 13C’60; and ‘2C’80;, formed by 584 A photoionization of CO,, have been measured by a photon-photoion coincidence technique. The corresponding 584 A photoelectron spectra, measured for ‘2C’602 and 13C’602, showed that the relative photoelectron branching ratios for forming the X2& and% 2X; states of CO; are isotope independent. The fluorescence quantum yields are greater than unity for the x2&, state and less than unity for E ’ S; for the three isotopic ions but the vaIues are isotopic dependent, th6 difference from unity correlating with the degree ofspectral perturbation in the 5 2Z; - x 211g 0: band. Calculations are made to identify the vibronic levels of ?i’lIu capable of interacting with the % 22: (0,0,O) level. The discrepancy concerning the apparent population ratio of the A21Tu and B 2& states of a;, formed by photoionization of CO,, between the results of photoelectron spectroscopy and those of CO; ion fluorescence intensities is explained as due to interelectronic state coupling_ The effects of this discrepancy on the determination of cross sections for forming the % ‘2: and x211,, states by various excitation mechanisms are discussed.
1. Introduction
that interelectronic state coupling can have only small effect on the distribution of the photoelectron spectral features but can considerably modify the optical emission spectra. In the case of CO;, if the 2 and x states perturb each other, this model leads us to expect shifts of emission intensity from the 2900 A region, traditionally considered to be E 2J?Zi+ % 211p(fig. l), to longer wavelength regions normally considered to be x21$, 4 z ‘lIg, as explained in ref. [6] _Counting of fluorescence photons in the 2800-2900 A region as % 22i --f g211g and in the 3000-5000 A region as X2D,,+Xsg would therefore lead to underestimatin&g th,e true 5-X emission yield and overestimating the A-X yield. Indeed the apparent quantum yield of fluorescentie @ (see section 2) from the x2Zl state has been shown by a direct measurement [S] to be significantly less than unity [a(%) = 0.54 + 0.131, whereas ihe apparent yield of the x2&, state exceeds unity [a(x) = I.8 * 0.15]. show
a very
The population ratio of the x2& and 5 2ZZzstates, of CO;, formed by 584 a photoionization of CO,, has been determined by the technique of photoelectron spectroscopy [l 1, which gives the distribution of ionic states. This ratio differs markedly from that inferred from the relative intensities of the x211u -+ % 211nand x “Zc + x 211gemissions of the CO; ion [2-51. Photoel&ron spectroscopy shows a greater initial population of g than of x (x/g = 0.65), whereas ion fluorescence ind@tes ari cperent greater popuIation of the emitting A state (A/B = 2.2). The suggestion that this discrepancy involves interelectronic state coupling between the ‘s *Xf and x*D,, states [l] has been studied by model calculations [6]. These calculations *Present address: Service de Physique du Solide et de RCsonance Magrkique, Orme des Merisiers, C.E.N. Saciay, B.P. 2, 91190 Gikur-Yvette, France.
114
S. Leach et al./FIuorescencequantumyields of isotopic CO; ions
tY&aT,
CO; EMISSION
h”58L
o-o,- M-
ii’
2800
-R2rrg
d-k!k 3000
--AA-,
3500
200
L500
Fig. 1_ Microdensitometer trace of the emission spectrum of CO; obtained by 100 eV electron impact on a CO2 jet [7]. Interstate
M*l=l
coupling could be affected by isotopic
substitution in a species such as CO: where, as we will demonstrate later, the density of interacting levels can be very small. This paper reports further fluorescence quantum yield and photoelectron spectroscopy measurements to examine whether isotopic substitution affects the z/E ratios and the discrepancy in the apparent state populations_
2. Fluorescence quantum yields Fluorescence quantum yields have been measured by a photon-photoion coincidence technique [8] briefly described previously [S] . A jet of gas M(= CO,) issuing from a hypodermic needle is crossed by a 1 mm beam of 584 a radiation from a helium resonance lamp (fig. 2). CO, pressure is = 2 X 10m5 torr in the main vessel and about 5 X IO-4 torr in the jet. Photoions Mf are formed in various electronic states i with branching ratios f;- given by photoelectron spectroscopy_ These ions are accelerated to an open multiplier ion detector
by an electric field of the order of 100 V/cm. The ion
fight times are in the microsecond range. Ions formed in excited electronic statesmay emit fluorescence photons, which can be detected by a photomuItipIier using filters for wavelength selection. The photoelectrons can also be detected by a channeltron detector, but without energy selection, enabling &hetime origin of ion flight times and photon emission times to be determined.
I-IOn-e-delay
Fig_ 2. Schema of photoion-electron coincidence experiments.
tim
I
and photoion-photon
Coincidence detectionand counting is made with a
system using TTL-Schottky fast logic circuits (fig. 3) whose inputs are the signals from fast discriminators that shape the signals from the detectors. True coincidences (T) and false coincidences (F) are counted simultaneously using a single coincidence gate which is opened twice. The first opening corresponds to a 1 to 7 cts variable delay with respect to the photon (or electron) signal, the second to a delay of 14 ps, so as to count ions completely uncorrelated with the initial photon. The coincidence gate open time A can be adjusted so as to modify the resolution time. The influence of experimental parameter drift is limited by counting either for a constant time or for a constant number of photons or of false coincidences. The coincidence counter is coupled to a D/A converter so that the coincidence spectrum can be traced out on a recorder. We first measure electron-ion coincidences; the delay fi between electron arrival and the opening of the coincidence gate is scanned, yielding a time-offight (TOF) mass spectrum. Each ion has its own flight time; ti the case of CO*, the ions observed are CO;, CO+ and O+ (fig. 4), the fragment ions being formed by predissociation of the ?!?Zi state at 19.3 eV [9,10]_ The 584 A (21.22 eV) exciting wavelength is insufficientIy energetic to form excited states of the fragment ions but can create the parent CO; ions in the fluorescingstates z2.Zi (18.18 eV) and x2& (17.31 eV). The coincidence gate is set on the CO; ion and photonion c&ncidence are accumulated. Fluorescence intensity measurements were carried
S. Leach et al./FIuorescence quantum yields of isotopic CO; ions
115
TTL COINCIDENCE
co;1
I-L DECAY,hd,
COINCIDENCE
ZOF MASS
co;
co’
\
Fig. 3. Electronic setup for coincidence detection and counting.
out by two different methods: (i) The coincidence signal is measured as a function of the ion-photon delay fd, with A at its minimum value of 20 ns. This gives the emission decay curve with itsrising edge centered on the flight time of the ion (fig. 4) from which the fluorescence lifetime rF can be determined; integration of the curve yields the fluorescence intensity. (ii) A is set to 385 ns, which is much greater than rF = 120 ns, so that an optimum number of coincidences can be counted. The value of td is adjusted so as to obtainamaximum number ofcounts. Method is less subject to statistical errors than (i), but requires good knowledge,of the emission decay curve in order to evaluate the ckrection to be made for not detecting those photons emitted at times greater than n = 385 ns and very much greater than rF = 120 ns. Fluorescence intensities are combined with the rate of detected ions to give the fluorescence efficiency n(i) which is the ratio of the number of photons emitted from_theith state with respect to the total number of CO; ions, regardless of the state in which they are formed, for a time duration defmed by one of the count modes previously described. The experiments were carried out on 12C16_02, 13~160, and 12C180,. The shape of the x + B decay curve was found20 bz the same for all thEe isotopic ions and gave r(A + B) = 120 + 10 ns; T(A) = 120 f 10 ns was obtained from the x state curves. This means
-f
Sa
0* -
.Ll
Fig. 4. (1) Photoion-photoelectron
and (2) photoion-fluorescence photon G!400-5000 A) coincidence curves for CO2 ionized by 584 A radiation,.A = 20 ns in both cases.
that sampling of emission with A = 385 ns measures the same fraction of emission in each case and makes the comparison of fluorescence quantum yields very reliable by this method. The values of n(j) were measured in succession for 12@80f the pairs K!C160;, 13~160; ar,d 12cl60;, 2The results given in table 1 were obtained using A = 385 ns (method ii). Results obtained by integration of the decay curves (method i) are in excellent agreement with those in table 1 but are statistically less reliable. In measuring n(x), a Schott WG 305 filter was used to eliminate the emission from E--E around 2900 A; this filter was replaced by a transparent Spectrosil disk, for_equralent surface reflexion, in measurements of r](A f B). Corrections were made for variations in optical transmission and photomultiplier response as a function of wavelength and non-detection of 4% of the emitted light with A = 385 ns. Emission decay from both E and x states of CO; has been found to be biexponential [12] but the long lifetime (> 2 ps) component represents less than 1% of the total emission and has been neglected here. Values of n(s) were obtained by difference: Q(B) = n(x + 5) - s(&. In the present experiments, the number of ionisation events counted in each experiment was about ten times greater than in the earlier work [S], reducing.tbe statistical error by a factor of three. Determination of the absolute value of q(j) requires
116
S. Leach et ai./Ftuorescence quantum iields
Table 1 Ekorescence
quantum yields of isotopic
CO; ions
Ion 12(9-$,
Q(X
+ B)
stP &) Q(x) a) Q(z) rl tP/ll (I) a) QO=rl(j)/fi
0.64 0.44 a.20 1.77 0.51 2.20
f f * c r c
0.01 0.01 0.02 0.11 0.06 0.30
13~160;
12p30;
0.64 f 0.01
0.64 0.40 0.24 1.61 0.61 1.67
0.37 i 0.01 0.27 f 0.02 1.49 -+ 0.09 0.69 r 0.07 1.37 r 0.15
(for symbols; see text); [Ill.
+ + f t t +
0.01 0.01 0.02 0.10 0.07 0.19
fA = 0.249 + 0.009
[11],fg=0.394c0.011
calibration of the apparatus in order to determine the detection efficiency of ions and, more directly important, of photons. In our earlier reported experiments, calibration was done with emission of ions for which it was reasonable to assume a fluorescence quantum yield of unity, in particular the Ni(B 2Z+ - X 2Z’) ion transition [5]. In the present experiments, fluorescence efficiencies were cakulated relative to the total emission from the E and x states of 12C160z (2400-5000 a) which was previously found [S] to have an ionic state quantum yield @(A + g) = 1. Isotope effects in CO; are thz co_nveniently displayed. A standard value of q(A f B) was derived for ‘“CO; from the relation 7(x + %) = @(x f E)fx+~ usingfx+E-= 0.64 (sum of the 584 A photoelectron bra&ring ratios for the x211 andx2Zi states) fromSamson [Ill. Th\ ionic state fluorescence yields Q(J) for the ions in theith state formed by photoionization were calculated from the relation a(i) = n(i)/& using the n(j) and fi values given in table I_ The fi values were con-’ sidered to be identical for the three isotopic species, following the results to be discussed in section 3. The error limits for the q(j) values in table 1 are twice the standard deviation for sets of repeated measurements. The uncertainties in thefi values of Samso_n [l l] wge taken into account in determining the @(A) and cP(B) values.
3. Photoelectron spectra Photoelectron branching ratios are known for 12COi
ofisotopic CO; ions
but not previously for the other two isotopic ions.Measurements of the branching ratiosfx and fi were therefore carried out for the isotopic species 13~160; to see to what extent these branching ratios are isotopically sensitive. Photoelectron measurements were not made on 12C180i. Photoelectron spectra were measured using a cylindrical mirror electron energy analyser [13] and a conventional helium resonance lamp iq an arrangement which accepts photoelectrons at x 50” to the light beam direction. The experimental conditions were chosen to allow the greatest possible precision in the comparison between 13C02 and 12C02, rather than in the absolute value of the x/g population ratio. Five main experimental problems had to be considered: (I] Resolution The energy-spacings between the (5,j.O) and (5,0,0) levels of A and the (0, 0,O) level of B are so small (tig_ 5) that these levels are not fully resolved unless high resolution is used, with concomitant poor signal strength. The (5,0,0) and (6,0,0) levels contribute only a small fraction of the total intensity of x, however, and furthermore we fmd no significant difference in Franck-Condon factors in this region between 12C02 and 13C02, for the partially resolved peaks. Incomplete resolution therefore contributes negligible uncertainty to the comparison of x/E ratio between 12C02 and 13C02. (2) Angular distribution di’erences The an@ar distribution parameters p for A (~-0.7 * 0.1) and B (-0.5 f 0.1) photoionization of 12C0, are quite different [14] so it is necessary either to allow for this or to use the magic angle (54” 44’) where angular effects vanish. There might be a difference between 12C02 and 13C02 in this respect if different degrees of configuration mixing are involved. so it is important that the present measurements are taken so near to the magic angIe that a very large change in p values between the isotopic species wouhJ b_eneeded to produce any significant effect on the A/B ratio comparison. We would consider this extremely unlikely. (3) IntenSity precision Intensity measurements are subject to statistical errors and to errors from slow drifts in lamp intensity, gas pressure and multiplier gain. All these sources of errors were minimized in the present measurement by making rapid repetitive scans over the photoelectron spectrum and accumulating . them for 10 mm for each substance. The gas pressure used was accurately the same for 12C02 and 13C02
S. Leach et aI.fFIuo.~escence quantum yielas of isotopic C@ ions
Ionization
i7.5
e~~rgyleVl=hv58~-Elil
17
Fig. 5.584 B. photoelectron spectra of 1*C’602 and 13C’602 in the ionization energy range 17 to 18.5 eV.
(2 X 10m5 torr in the main chamber), and was sufficiently low to eliminate paralysis effects. Repeated measurements were made. The effectiveness of these precautions was checked by examining the consistency of different measurements of 12C02 and r3C02 photoelectron spectra among themselves: the differences remaining were ascribable to statistical uncertainty only. 14) Background subtraction The most serious difficulty in the way of obtaining an absolute x/E ratio from the present experiments is the scattered electron background. Photoelectron peaks have a weak tail to low electron energies, on which @her peaks are superimposed. Precise subtraction of this background can be a major problem. In the present work the problem is largely avoided by taking the difference between 1*C02 and 13C02 spectra, measured under identical conditions. It is very improbable that changes in the background between 12C02 and t3C02 would compensate for any changes in the peaks; the result given below refers to the difference between spectra including background.
117
(5) Electron energy discriminations Since the photoelectron energies for 12C02 and 13C02 are identically the same, discrimination effects in the electron analyser have no effect on the comparison between isotopic species. It is favorable to the determination of absolute A/E population ratios that the present analyser contains no electron lenses, so that its energy bandpass, proportional to energy, controls the transmission function. The two best runs each on 12C02 and 13C02 were selected for analysis. The accumulated counts (including background) were summed wIthinJhe ranges of each of the (000) and (100) peaks of B and the (OOO)(400) peaks ofx;, and the figures so obtained were normalized to a constant total sum for 12C02 and 13C02 before being compared. No difference exceeded two standard deviations of the counting statistics In any case. Therefore, no difference in Franck-Condon factors between 1*C02 and 13C0 could be discerned in these data (fig. 5). Next, total H and x intensities in the normalized spectra yre derived as sums over the two-chosen peaks from B and the five chosen peaks of A, and the figures for 12C02 and 13C02 were compared_ The_differLnces between them divided by the totals for A and B (with approximate background subtraction) were combined to determine the percentage difference in the ratio x/x between the two isotopes. The result was a difference of -0.1 ? l.OS%, which is a null value. In order to obtain an approximate value for the absolute x/z ratio, the missing levels of both x and 5 were added in using published photoelectron spectra taken at higher resolution. Our preferred result is x/E = 0.625 but there is no simple way to estimate its accuracy, since it depends on the correctness of the background subtraction. The background was estimated in three ways: (1) Separately for each peak, as ro&tly the mean of locally occurring valleys. Result: A/B = 0.625. (2) As constant over x re@n and a different constant over B region. Result: A/g = 0.629. (3) Constant over the whole spectrum_ Result: x/E = 0.585. Some previous results for the X/E ratio in 12C160s are0.65 [1],0.69+0.12 [15],0.63 +0.04 [ll]. The main result of the phzto$ectron spectral measurements is that the initial A/B population ratios m photoionization of 13C02 and ‘*CO, at 584 A are
118
S. Leach et ai./FIuorescencequantum yields of isotopib CO: ions
3dentical within 1%. In view of this result we expect aiso a negligible isotope effect on the x/z ratio in -12ClSo;_
4. Discussion It is clear from table 1 that isotopic substitut.Jon has a m_arked effect on q(j) and on the derived Q(A) and a(B) values. The emission intensity is considerably greater in the 3000-5000 A region (apparent X-Z) than in the 2900 A region (apparent X-2) for all three isotopic ions. However, the discrepancy with respect to the x and % state populations determined from the photoelectron spectra is smaller in 12C180; than in 12C160i, and further reduced in 13,CO$_The fluorescence quantum yields of the A and B states in the heavier ions are consequently closer to unity with respect to the situation in 12CO$. According to the model discussed briefly in section 1, and in detail elsewhere [6], the results presented above indicate -hat a shift of emission intensity occurs from the 2800-2900 A region to the 3000%-5000 A region, due to interelectronic state coupling between A21T, and B 2Zz, but that its extent decreases iu the order 12~160: > 12~180; > 13C160i. It is now of interest to discuss spectroscopic information on interelectronic coupling in CO; and this is done in the following subsection_ 4.1. IntereieHronic
State coupting in CO;
Interelectronic state coupling in a species as small as CO; can manifest itself as spectral perturbations ]16] _Cauyacq et al. [17J have carried out a high resolution study of the E 2Zc - ? 211g(0, 0,O) - (0, 0,O) band of 12C160z. Rotational analysis of this band showed the existence of a small perturbation around J’ = 36.5, previously observed by Bueso-Sanllehi [18], and also, for the first time, a strong perturbation at levels below J’ = 20.5. Fig. 6 gives the scaIed rotational term values ?(N)0.37N(N f 1) as a function of N(N + 1) for the 5 22i (0, 0,O) level. The 0.37 scaling factor was chosen to cIearly reveal spin-splitting anomalies and the perturbation features_ Avoided level crossing between the f rotational levels* of the perturbed E 2Ci state and the unknown perturbing state En occurs in the neighbour-
j
\+
,doo
,
.i” :.:f 2uoo
t4
,
1
N+1)300”
Fig. 6. 2 *Zt state of CO;. Scaled term values T(N) 0.37N(N + 1) as a function ofN(N + 1). Becauseof the scale expansion the 13 rotational constant of the perturbing state appears negative.
hood ofJ’ = 16.5. These f levels are displaced by up to 15 cm-l. The e levels of the 3 2Zi(0, 0,O) state are also perturbed at low J’ values, with maximum displacement of 3 cm-l. The perturbing state Ep lies at about 34620 cm-l above the “x 2111g ground state, i.e. about 20 cm-l above E2ZBi (0, 0,O) (fig_ 6). Avoided level crossing can occur because analysis shows that the E state has a smaller rotational constant than the 522f(O, 0,O) level. Ep is therefore about 6120 crn_-l above the mean enerOv of the x 2TI1,2(0, 0,O) and A 21T,,2(0, 0,O) levels (the spin orbit split’tig of the “A211u state is about 96 cm-l). The low-lying excited states likely to be in the neighbourhood of the E 2Zz(0, 0,O) level are (in linear geome-
levels with parity +(-l#+s-l are e levels,.those w&h parity -(-l)?S-l are f levels. In the % ‘2: state of. CO;, the F1 levels fJ’ = N’ + l/2) are alle, the Fa levels (J’ ‘N’ - l/2) aref. Selection rules forperturbation are AJ= 0,e*e,f*f,e+f.
* Rotational
S. Leachetal./Fluorescencequantumyieldsof isotopicCOf ions
tly) .. . 302, 17r417r250 %- [19], . ..34 1Tr; 17r; 27r”4rIi [20] and .. . 3:; 1:: 14 fill=. In their linear configu-
rations the energiesof the two-quartet states are calculated to be at least 1 eV above the !? 2Zi(0, 0,O) level ]19,20]. The x *Tin state is therefore the best candidate for assigning the En state. The electronic configu rations would in any case favour a first order interaction of B *2: with the x *Ru state rather than a higher order interaction with either the 4Z; or a&, state which differ from E *xi by two or more spin orbitals. Potentjal energy surface caIcuIat&rs of the __.30, 1~: lrz B*Zi and _._30: 1~: 1x2 A*&, states were carried out by Horsley [ 191. These calculations were made with molecular wavefunctions represented by a linear combination of 350 configurations constructed from a molecular orbital basis set. Linear combinations of exponential function orbitals, centered on the atoms, with SCF optimized coefficients, were used toconstruct the molecular orbitals. The x *I$, potential energy surface was calculated to intersect With the 5 *Zi state at the (0, 0,O) level of the latter (fig. 7) thus supporting the assignment of x *II as the perturbing state E . Inrerelectronic state couplin~must involve a vibronic level of the x 211Ustate isoenergetic with the % *LB: (0, 0, 0) level and of appropriate symmetry.
E(eVl
-13-
-15
-17 -
-19 -
-21.
, 1
1
r&Al 2
Fig. 7. Potential energy of the B2ZL and ~zl$ states of COf calculatedas a function of r(C0) distance, The energy scale is relative to the energy of the CO? ion.
119
The expression: WR,]G,(u,,
u*, u3)
where G$ = vi -xi, was used to calculate approximate energy levels of the linear x ?-II,, state in order to determine which combinations of its three vibrational modes can form a suitable vibronic level E, isoenergetic with E2Zi(0, 0,O). The symmetry requirements for interaction impose that E, be an ungerade vibronic state. The vibrational symmetries are respectively vl($), v2(fI,), v3(Zz) and the electronic state symmetry of Ep is I$, SO that the product of representations I’(&,) X I’(ul) X I?&) X I’(+) = f’(Ep) implies u2 f u3 = even for E to have ungerade symmetry. WKen the symme’try requirements are satisfied two cases can arise: (a) The two interacting levels may have the same vibrational angular momentum quantum number I (same vibronic species). (b) The two interacting levels may have AI = f 1. Higher Al values are forbidden. AnaIysis of the x 211,, vibrational levels, although progressing [21], is insufficiently advanced to provide complete precise information about anharmonicities, Renner-Teller and Fermi interactions invohing several vibrational quanta. The calculations were therefore done using the following constants for the “A*lIu stateoff2C’60~:,1 = 1131 cm-l [22],u2=440 cm-l [21],~3 =2731 cm-l 231;~;~ a-3 cm-1 i = [221,x& = -1 cm-l [21],xS3 = -10 cm-l;x& 5 cm-l, XL = -20 cm-r x0 = -10 cm-l. The ?I3were chosen arbitrarivalues ofx!j3, xy2 ,xt3 and’ x23 ly but are of the order and sign expected in CO; “A*II,, from comparison with known values for the B02 x *II,, state [24] and the CO2 % 1Zi state [25]. It should benoted that with ET = 6120 cm-l, not more than two quanta of us or five quanta of v1 can participate in the perturbing level. As is clear from eg. (I), Fermi resonance and coupling due to vibrational angular momentum, in particular the Renner-Teller effects, were not explicitly considered in our En calculations_ Instead it was assumed that these interactions give rise to sets of vibromc levels ofdifferingK(K=I~hCII;I=u2,u2 -2, _._1 or0)
whose energy spread will span the region ofEp = 6120 cm-l if the zero-order calculated level is within 300 cm-l of E,. This is a reasonable limit if the number of v2 quanta is not too high. The following vibronic levels of the daft,, state were thus calculated to lie within 6120 + 300 cm-l and to have the necessary ungerade symmetry (i.e. v2 f v3 = even) for interaction with the E 22z O” level; (0,147 O), (1, 5, I), (0,2,2), (326, O), (2,3>1) and (4,4,0). To achieve this interaction, the (0,14,0), (0, 2,2), (3,6,0) and (4,4,0) levels can form K = 1 (If,) vibronic states, w,hile (1,5, 1) and (2,3, 1) can form K = 0 (Xi) states. .Fig. 6 shows that the E 21Zi (0, 0.0) e and f rotational levels are perturbed differently at low Nvalues. A *Zt vibronic perturbing state (perturbation heterogeneous in I, homogeneous in K) would be expected to affect about equally the e and f levels of a particuIarNvahte of the E21ZL perturbed state since the spin splitting is relatively small at low N values. Marked differences in perturbation of the e and f levels are more characteristic of interaction with a ‘II, vibronic state (perturbation homogeneous in I, heterogeneous in K) whose Q = l/2 and 52 = 312 levels would perturb different N levels. From this we conclude that the four levels (0, 14,0), (0,2,2), (3,6,0) and (4,4,0) are possible perturbing states. The Franck-Condon factors are not likely to be large for the (0,14,0) level. The most probable candidates for En are therefore the D, Renner-Teller vibronic states (eventually modified by Fermi resonance interactions) issuing From the (0,2,2), (3,6,0) and (4,4,0) vibrational levels of x 2ffu. Detailed optical spectroscopic analysis in progress should yield further information on the nature of the observed perturbations. The sparseness of the possible perturbing levels, and the resonance limit type interaction as revealed by the rotational perturbations, make it conceivable that isotopic substitution could markedly affect the interelectronic state perturbation. This is demonstrated not only by the isotopic eft&; of fluorescence quantum yields of the x211U and B Z, states (table l), but also in the spectral perturbations. Work in progress [26] on the analysis of the % 28t - 2 211p0: band of the isotopic species 13C1601 and 12C180$ indicates that, as compared with 12C160;, spectral perturbations are less severe in 12C1gOi and even less so in 13~160$
This behaviour correlates with the quantum yield trends. Further quantitative data on interelectronic state coupling for the isotopic CO; ions requires completion of the high resolution spectral analyses under way-
5. Conclusion In this work we have shown that the photon emission yields of the x2& and % 2Ci states of the CO; ion are markedly isotope sensitivk, whereas the x/E population ratios as determined from 584Aphotoelec tron spectroscopy are identical for the pair of isotopic species 12C1602, 13Ci60,. These results can be interpreted in terms of a model [6] which shows that interelectronic coupling between the E 2Zi and x 2fIu states of CO; can give rise: (a) to a significant shift of photon emission intensity from the spectral region usually considered to be x 2,~: - g 2,“s (2800-2900 li), to that considered to be A 2nd - X2$ (3000-5000 A) ‘; (b) to only a very limited spectral redistribution in the photoelectron spectrum. Our interpretation also requires interelectronic coupling to be isotope sensitive in CO;. The observation that the degree of spectral perturbations in the isotopic ions correlates with the trend of the quantum yields away from unity supports the above interpretation. We conclude, furthermore, that interelectronic interactions explain the much discussed discrepancy in
the apparent population ratio of the x 2ffu and % 22: states of CO; ions, formed by 584 A photoionization of C02, between the results of photoelectron spectroscopy [I,1 1,15] and those of photon emission from the ion states [2-S]_ This discrepancy is implicitly important in a number of physical situations, some of which we will briefly discuss. The first situation concerns the Martian dayglow in which -2B Z,f -X-2 f$andA -2 ff, -X-2 f&emissions of CO; are important features. Airglow models involve
* Direct confmation of a shift in emission intensity from the 2900
A to the 30004000
A region in ‘2C’60~
is given by
recent threshold photoeIectron-photoion coincidence experiments on CO2 In which photon emission was observed at A > 3300 A In coincidence with CO; ions formed in the B%;(O, 0,O) level [30].
interpretation of the intensides of these emissions and this has been done in terms of photoionization, fluorescent scattering and photoelectron impact mechanisms [27-29 J. Interelectronic coupling was ignored in these interpretations. Effects on the results of Martian airglow model calculations, when B *Xix *If,, coupling is specifically taken into account, are discussed elsewhere [3 11. Other cases where interelectronic coupling between the emitting states of CO; should be taken into account concern the determination of the cross sections for formation of the 3 2Xi and x %Iu states of CO; by other than photon impact. These include fast particle collisions with CO2 [32,33] and Penning‘[34] and electron impact [35,36] ionization of CO,. In each of these cases, the ratio of the emission intensities in the 2800-2900 a and 3000-5000 A regions was considered to be directly proportional to the ratio of the g *Xi and ;\‘*lI,, state populations. We believe that intereiectronic coupling makes this invalid, as we have demonstrated above. As a final example where the CO; ion excited state apparent population discrepancy can lead to doubtful interpretations, we mention the work of Strickland and Green [37] on electron impact cross sections for CO?. In this, the population discrepancy, taken at its face value, suggested to these authors that the relative magnitudes of the optical oscillator strength for the E *Xi and x *II,, ionization continua might differ from those of the generalized oscillator strengths. Our results indicate that this is an unnecessary consideration in this particular instance. Acknowledgement We wish to thank D. Gauyacq, J.A. Horsley, J.P. Maier and J.A.R. Samson for making available unpublished results. References [ 11 J.A.R. Samson and J.L. Gardner, I. Geophys. Res. 78 (1973) 3663. [2]
T.S. Wauchop and H.P. Broida, J. Geophys. Res. 76
(1971) 21. [3] L.C. Lee and D.L. Judge, J. Chem. Phys. 57 (1972) 4443. [4] E.P. Gentieu and I.E. Mentalt, J. Chem. Phys. 64 (1976) 1376. [S] J.&D. Eland, M. Devoret and S. Leach, Chem. Phys. Letters 43 (1976) 97.
[6] S. Leach, P.S. Stannard and W.M. Gelbart, to be published in Mol. Phys. [7] M. Homni and S. Leach, Compte-Rendus AC. Sci. (Paris)
248 (1959) 2196; M. Horani and S. Leach, J. Chtm. Phys. 58 (1961) 825. [8] M. Devoret, These 3eme Cycle, Orsay (1976). [9] J.H.D. Eland, Intern. I. Mass Spectry. Ion Phys. 9 (1972) 397. [lo] J.&D. Eland and J. Berkowitz, I. Chem. Phys. 67 (1977) 2782. [Ill J.A.R. Samson, private communication (1977). [12] J.P. Maier, private communication (1977). [13] I. Berkowitz, J. Chem. Phys. 56 (1972) 2766. [14] T.A. Carlson, GE. McGuire, A.E. Jonas, K.L. Cheng, C-P. Anderson, C.C. Lu and B.P. PuBen, in: Electronic spectroscopy, ed. D.A. Shirley, (North-Holland, Amsterdam, 1972) p_ 207. [15] J.A.R. Samson, J.L. Gardner and I.E. MentaB, I. Geophys. Res., 77 (1972) 5560. [16] S. Leach, in: Molecular energy transfer, eds. R.D. Levine and J. Jortner (Halsted Press - Wiley, New York, 1975) p. 163. [17] D. Gauyacq, M. Honni, S. Leach and J. Rostas> Can. J. Phys. 53 (1975) 2040. [18] F. Bueso-San&hi, Phys. Rev. 60 (1941) 556. [19] I. Horsley, unpublished work, Orsay (1974). [20] W.B. Enghmd, B.J. Rosenberg, P.J. Fortune and A.C. Wahl. J. Chem. Phys. 65 (1976) 684. [21] C. Bnnciard-Larcher, D. Gauyacq and J. Rostas, 21st International Colloquium on Astrophysics, Liege, June 1977, to be published; D. Gauyacq and J. Rostas, to be published [22] S. Mrozowski, Phys. Rev. 62 (1942) 270. 1231 S. Mrozowski, Phys. Rev. 72 (1947) 691. [24] J.W.C. Johns, Can. I. Phys. 39 (1961) 1738. [25] C.P. Courtoy, Ann. Sot. Sci. (Bruseties) 73 (1959) 5. 1261 D. Gauyacq, work in progress. [27] A. Datgarno, T.C. D_eggesand A.I. Stewart, Science 167 (1970) 1490. [28] A. Dalgarno and T.C. Degges, in: Planetary Atmospheres, I.A.U. Symposium No 40, eds. C. Sagan, T.C. Owen and H.J. Smith (Springer, Berlin, 1971) p. 337. [29] A.I. Stewart, J. Ceophys. Res. 77 (1972) 54. [30] E.W_ S&tag. R. Frey, B. Gotchev, W.B. Peatman and H. PoBak, Chem. Phys. Letters 51 (1977) 406. [31] S. Leach, 21st International Colloquium on Astrophysics, Lisge, June 1977, to be published.’ [32] M.A. Coplan and J.E. blentall, J. Chem. Phys. 58 (1973) 4912. [33] M.A. Haugh and J.R Birely, J. Chem. Phys. 60 (1974) 164. [34] T.S. Wauchop and H.P. Broida, J. Quant. Spectry. Rad. Transfer 12 (1972) 371. [35] J.W. McConkey, D.J. Bums and J.M. Woolsey, J. Phys. Bl (1968) 71. [36] J-M. AjeBo, J. Chem. Phys. 55 (1971) 3169. [37] D.J. Strickland and A.E.S. Green, J. Geophys. Res. 74 (1969) 6415.
FLUORESCENCE QUANTUM YIELDS OF ISOTOPIC CO; IONS Sydney LEACH, Michel DEVORET* Lnboratoire de Photophysique 91405 - Orsay. France
Molhlaire
du C.N.R.S., Britiment 213, Universit6 de Pariis-Sud,
and John
H.D. ELAND
Argonne National Laboratory, Argonne, 11&z&s 60439, USA Received 24 February
1978
Fiuorescence quantum yields of the x ‘II~ and 6 ‘$ states of the isotopic ions 1*C’60;, 13C’60; and ‘2C’80;, formed by 584 A photoionization of CO,, have been measured by a photon-photoion coincidence technique. The corresponding 584 A photoelectron spectra, measured for ‘2C’602 and 13C’602, showed that the relative photoelectron branching ratios for forming the X2& and% 2X; states of CO; are isotope independent. The fluorescence quantum yields are greater than unity for the x2&, state and less than unity for E ’ S; for the three isotopic ions but the vaIues are isotopic dependent, th6 difference from unity correlating with the degree ofspectral perturbation in the 5 2Z; - x 211g 0: band. Calculations are made to identify the vibronic levels of ?i’lIu capable of interacting with the % 22: (0,0,O) level. The discrepancy concerning the apparent population ratio of the A21Tu and B 2& states of a;, formed by photoionization of CO,, between the results of photoelectron spectroscopy and those of CO; ion fluorescence intensities is explained as due to interelectronic state coupling_ The effects of this discrepancy on the determination of cross sections for forming the % ‘2: and x211,, states by various excitation mechanisms are discussed.
1. Introduction
that interelectronic state coupling can have only small effect on the distribution of the photoelectron spectral features but can considerably modify the optical emission spectra. In the case of CO;, if the 2 and x states perturb each other, this model leads us to expect shifts of emission intensity from the 2900 A region, traditionally considered to be E 2J?Zi+ % 211p(fig. l), to longer wavelength regions normally considered to be x21$, 4 z ‘lIg, as explained in ref. [6] _Counting of fluorescence photons in the 2800-2900 A region as % 22i --f g211g and in the 3000-5000 A region as X2D,,+Xsg would therefore lead to underestimatin&g th,e true 5-X emission yield and overestimating the A-X yield. Indeed the apparent quantum yield of fluorescentie @ (see section 2) from the x2Zl state has been shown by a direct measurement [S] to be significantly less than unity [a(%) = 0.54 + 0.131, whereas ihe apparent yield of the x2&, state exceeds unity [a(x) = I.8 * 0.15]. show
a very
The population ratio of the x2& and 5 2ZZzstates, of CO;, formed by 584 a photoionization of CO,, has been determined by the technique of photoelectron spectroscopy [l 1, which gives the distribution of ionic states. This ratio differs markedly from that inferred from the relative intensities of the x211u -+ % 211nand x “Zc + x 211gemissions of the CO; ion [2-51. Photoel&ron spectroscopy shows a greater initial population of g than of x (x/g = 0.65), whereas ion fluorescence ind@tes ari cperent greater popuIation of the emitting A state (A/B = 2.2). The suggestion that this discrepancy involves interelectronic state coupling between the ‘s *Xf and x*D,, states [l] has been studied by model calculations [6]. These calculations *Present address: Service de Physique du Solide et de RCsonance Magrkique, Orme des Merisiers, C.E.N. Saciay, B.P. 2, 91190 Gikur-Yvette, France.
114
S. Leach et al./FIuorescencequantumyields of isotopic CO; ions
tY&aT,
CO; EMISSION
h”58L
o-o,- M-
ii’
2800
-R2rrg
d-k!k 3000
--AA-,
3500
200
L500
Fig. 1_ Microdensitometer trace of the emission spectrum of CO; obtained by 100 eV electron impact on a CO2 jet [7]. Interstate
M*l=l
coupling could be affected by isotopic
substitution in a species such as CO: where, as we will demonstrate later, the density of interacting levels can be very small. This paper reports further fluorescence quantum yield and photoelectron spectroscopy measurements to examine whether isotopic substitution affects the z/E ratios and the discrepancy in the apparent state populations_
2. Fluorescence quantum yields Fluorescence quantum yields have been measured by a photon-photoion coincidence technique [8] briefly described previously [S] . A jet of gas M(= CO,) issuing from a hypodermic needle is crossed by a 1 mm beam of 584 a radiation from a helium resonance lamp (fig. 2). CO, pressure is = 2 X 10m5 torr in the main vessel and about 5 X IO-4 torr in the jet. Photoions Mf are formed in various electronic states i with branching ratios f;- given by photoelectron spectroscopy_ These ions are accelerated to an open multiplier ion detector
by an electric field of the order of 100 V/cm. The ion
fight times are in the microsecond range. Ions formed in excited electronic statesmay emit fluorescence photons, which can be detected by a photomuItipIier using filters for wavelength selection. The photoelectrons can also be detected by a channeltron detector, but without energy selection, enabling &hetime origin of ion flight times and photon emission times to be determined.
I-IOn-e-delay
Fig_ 2. Schema of photoion-electron coincidence experiments.
tim
I
and photoion-photon
Coincidence detectionand counting is made with a
system using TTL-Schottky fast logic circuits (fig. 3) whose inputs are the signals from fast discriminators that shape the signals from the detectors. True coincidences (T) and false coincidences (F) are counted simultaneously using a single coincidence gate which is opened twice. The first opening corresponds to a 1 to 7 cts variable delay with respect to the photon (or electron) signal, the second to a delay of 14 ps, so as to count ions completely uncorrelated with the initial photon. The coincidence gate open time A can be adjusted so as to modify the resolution time. The influence of experimental parameter drift is limited by counting either for a constant time or for a constant number of photons or of false coincidences. The coincidence counter is coupled to a D/A converter so that the coincidence spectrum can be traced out on a recorder. We first measure electron-ion coincidences; the delay fi between electron arrival and the opening of the coincidence gate is scanned, yielding a time-offight (TOF) mass spectrum. Each ion has its own flight time; ti the case of CO*, the ions observed are CO;, CO+ and O+ (fig. 4), the fragment ions being formed by predissociation of the ?!?Zi state at 19.3 eV [9,10]_ The 584 A (21.22 eV) exciting wavelength is insufficientIy energetic to form excited states of the fragment ions but can create the parent CO; ions in the fluorescingstates z2.Zi (18.18 eV) and x2& (17.31 eV). The coincidence gate is set on the CO; ion and photonion c&ncidence are accumulated. Fluorescence intensity measurements were carried
S. Leach et al./FIuorescence quantum yields of isotopic CO; ions
115
TTL COINCIDENCE
co;1
I-L DECAY,hd,
COINCIDENCE
ZOF MASS
co;
co’
\
Fig. 3. Electronic setup for coincidence detection and counting.
out by two different methods: (i) The coincidence signal is measured as a function of the ion-photon delay fd, with A at its minimum value of 20 ns. This gives the emission decay curve with itsrising edge centered on the flight time of the ion (fig. 4) from which the fluorescence lifetime rF can be determined; integration of the curve yields the fluorescence intensity. (ii) A is set to 385 ns, which is much greater than rF = 120 ns, so that an optimum number of coincidences can be counted. The value of td is adjusted so as to obtainamaximum number ofcounts. Method is less subject to statistical errors than (i), but requires good knowledge,of the emission decay curve in order to evaluate the ckrection to be made for not detecting those photons emitted at times greater than n = 385 ns and very much greater than rF = 120 ns. Fluorescence intensities are combined with the rate of detected ions to give the fluorescence efficiency n(i) which is the ratio of the number of photons emitted from_theith state with respect to the total number of CO; ions, regardless of the state in which they are formed, for a time duration defmed by one of the count modes previously described. The experiments were carried out on 12C16_02, 13~160, and 12C180,. The shape of the x + B decay curve was found20 bz the same for all thEe isotopic ions and gave r(A + B) = 120 + 10 ns; T(A) = 120 f 10 ns was obtained from the x state curves. This means
-f
Sa
0* -
.Ll
Fig. 4. (1) Photoion-photoelectron
and (2) photoion-fluorescence photon G!400-5000 A) coincidence curves for CO2 ionized by 584 A radiation,.A = 20 ns in both cases.
that sampling of emission with A = 385 ns measures the same fraction of emission in each case and makes the comparison of fluorescence quantum yields very reliable by this method. The values of n(j) were measured in succession for 12@80f the pairs K!C160;, 13~160; ar,d 12cl60;, 2The results given in table 1 were obtained using A = 385 ns (method ii). Results obtained by integration of the decay curves (method i) are in excellent agreement with those in table 1 but are statistically less reliable. In measuring n(x), a Schott WG 305 filter was used to eliminate the emission from E--E around 2900 A; this filter was replaced by a transparent Spectrosil disk, for_equralent surface reflexion, in measurements of r](A f B). Corrections were made for variations in optical transmission and photomultiplier response as a function of wavelength and non-detection of 4% of the emitted light with A = 385 ns. Emission decay from both E and x states of CO; has been found to be biexponential [12] but the long lifetime (> 2 ps) component represents less than 1% of the total emission and has been neglected here. Values of n(s) were obtained by difference: Q(B) = n(x + 5) - s(&. In the present experiments, the number of ionisation events counted in each experiment was about ten times greater than in the earlier work [S], reducing.tbe statistical error by a factor of three. Determination of the absolute value of q(j) requires
116
S. Leach et ai./Ftuorescence quantum iields
Table 1 Ekorescence
quantum yields of isotopic
CO; ions
Ion 12(9-$,
Q(X
+ B)
stP &) Q(x) a) Q(z) rl tP/ll (I) a) QO=rl(j)/fi
0.64 0.44 a.20 1.77 0.51 2.20
f f * c r c
0.01 0.01 0.02 0.11 0.06 0.30
13~160;
12p30;
0.64 f 0.01
0.64 0.40 0.24 1.61 0.61 1.67
0.37 i 0.01 0.27 f 0.02 1.49 -+ 0.09 0.69 r 0.07 1.37 r 0.15
(for symbols; see text); [Ill.
+ + f t t +
0.01 0.01 0.02 0.10 0.07 0.19
fA = 0.249 + 0.009
[11],fg=0.394c0.011
calibration of the apparatus in order to determine the detection efficiency of ions and, more directly important, of photons. In our earlier reported experiments, calibration was done with emission of ions for which it was reasonable to assume a fluorescence quantum yield of unity, in particular the Ni(B 2Z+ - X 2Z’) ion transition [5]. In the present experiments, fluorescence efficiencies were cakulated relative to the total emission from the E and x states of 12C160z (2400-5000 a) which was previously found [S] to have an ionic state quantum yield @(A + g) = 1. Isotope effects in CO; are thz co_nveniently displayed. A standard value of q(A f B) was derived for ‘“CO; from the relation 7(x + %) = @(x f E)fx+~ usingfx+E-= 0.64 (sum of the 584 A photoelectron bra&ring ratios for the x211 andx2Zi states) fromSamson [Ill. Th\ ionic state fluorescence yields Q(J) for the ions in theith state formed by photoionization were calculated from the relation a(i) = n(i)/& using the n(j) and fi values given in table I_ The fi values were con-’ sidered to be identical for the three isotopic species, following the results to be discussed in section 3. The error limits for the q(j) values in table 1 are twice the standard deviation for sets of repeated measurements. The uncertainties in thefi values of Samso_n [l l] wge taken into account in determining the @(A) and cP(B) values.
3. Photoelectron spectra Photoelectron branching ratios are known for 12COi
ofisotopic CO; ions
but not previously for the other two isotopic ions.Measurements of the branching ratiosfx and fi were therefore carried out for the isotopic species 13~160; to see to what extent these branching ratios are isotopically sensitive. Photoelectron measurements were not made on 12C180i. Photoelectron spectra were measured using a cylindrical mirror electron energy analyser [13] and a conventional helium resonance lamp iq an arrangement which accepts photoelectrons at x 50” to the light beam direction. The experimental conditions were chosen to allow the greatest possible precision in the comparison between 13C02 and 12C02, rather than in the absolute value of the x/g population ratio. Five main experimental problems had to be considered: (I] Resolution The energy-spacings between the (5,j.O) and (5,0,0) levels of A and the (0, 0,O) level of B are so small (tig_ 5) that these levels are not fully resolved unless high resolution is used, with concomitant poor signal strength. The (5,0,0) and (6,0,0) levels contribute only a small fraction of the total intensity of x, however, and furthermore we fmd no significant difference in Franck-Condon factors in this region between 12C02 and 13C02, for the partially resolved peaks. Incomplete resolution therefore contributes negligible uncertainty to the comparison of x/E ratio between 12C02 and 13C02. (2) Angular distribution di’erences The an@ar distribution parameters p for A (~-0.7 * 0.1) and B (-0.5 f 0.1) photoionization of 12C0, are quite different [14] so it is necessary either to allow for this or to use the magic angle (54” 44’) where angular effects vanish. There might be a difference between 12C02 and 13C02 in this respect if different degrees of configuration mixing are involved. so it is important that the present measurements are taken so near to the magic angIe that a very large change in p values between the isotopic species wouhJ b_eneeded to produce any significant effect on the A/B ratio comparison. We would consider this extremely unlikely. (3) IntenSity precision Intensity measurements are subject to statistical errors and to errors from slow drifts in lamp intensity, gas pressure and multiplier gain. All these sources of errors were minimized in the present measurement by making rapid repetitive scans over the photoelectron spectrum and accumulating . them for 10 mm for each substance. The gas pressure used was accurately the same for 12C02 and 13C02
S. Leach et aI.fFIuo.~escence quantum yielas of isotopic C@ ions
Ionization
i7.5
e~~rgyleVl=hv58~-Elil
17
Fig. 5.584 B. photoelectron spectra of 1*C’602 and 13C’602 in the ionization energy range 17 to 18.5 eV.
(2 X 10m5 torr in the main chamber), and was sufficiently low to eliminate paralysis effects. Repeated measurements were made. The effectiveness of these precautions was checked by examining the consistency of different measurements of 12C02 and r3C02 photoelectron spectra among themselves: the differences remaining were ascribable to statistical uncertainty only. 14) Background subtraction The most serious difficulty in the way of obtaining an absolute x/E ratio from the present experiments is the scattered electron background. Photoelectron peaks have a weak tail to low electron energies, on which @her peaks are superimposed. Precise subtraction of this background can be a major problem. In the present work the problem is largely avoided by taking the difference between 1*C02 and 13C02 spectra, measured under identical conditions. It is very improbable that changes in the background between 12C02 and t3C02 would compensate for any changes in the peaks; the result given below refers to the difference between spectra including background.
117
(5) Electron energy discriminations Since the photoelectron energies for 12C02 and 13C02 are identically the same, discrimination effects in the electron analyser have no effect on the comparison between isotopic species. It is favorable to the determination of absolute A/E population ratios that the present analyser contains no electron lenses, so that its energy bandpass, proportional to energy, controls the transmission function. The two best runs each on 12C02 and 13C02 were selected for analysis. The accumulated counts (including background) were summed wIthinJhe ranges of each of the (000) and (100) peaks of B and the (OOO)(400) peaks ofx;, and the figures so obtained were normalized to a constant total sum for 12C02 and 13C02 before being compared. No difference exceeded two standard deviations of the counting statistics In any case. Therefore, no difference in Franck-Condon factors between 1*C02 and 13C0 could be discerned in these data (fig. 5). Next, total H and x intensities in the normalized spectra yre derived as sums over the two-chosen peaks from B and the five chosen peaks of A, and the figures for 12C02 and 13C02 were compared_ The_differLnces between them divided by the totals for A and B (with approximate background subtraction) were combined to determine the percentage difference in the ratio x/x between the two isotopes. The result was a difference of -0.1 ? l.OS%, which is a null value. In order to obtain an approximate value for the absolute x/z ratio, the missing levels of both x and 5 were added in using published photoelectron spectra taken at higher resolution. Our preferred result is x/E = 0.625 but there is no simple way to estimate its accuracy, since it depends on the correctness of the background subtraction. The background was estimated in three ways: (1) Separately for each peak, as ro&tly the mean of locally occurring valleys. Result: A/B = 0.625. (2) As constant over x re@n and a different constant over B region. Result: A/g = 0.629. (3) Constant over the whole spectrum_ Result: x/E = 0.585. Some previous results for the X/E ratio in 12C160s are0.65 [1],0.69+0.12 [15],0.63 +0.04 [ll]. The main result of the phzto$ectron spectral measurements is that the initial A/B population ratios m photoionization of 13C02 and ‘*CO, at 584 A are
118
S. Leach et ai./FIuorescencequantum yields of isotopib CO: ions
3dentical within 1%. In view of this result we expect aiso a negligible isotope effect on the x/z ratio in -12ClSo;_
4. Discussion It is clear from table 1 that isotopic substitut.Jon has a m_arked effect on q(j) and on the derived Q(A) and a(B) values. The emission intensity is considerably greater in the 3000-5000 A region (apparent X-Z) than in the 2900 A region (apparent X-2) for all three isotopic ions. However, the discrepancy with respect to the x and % state populations determined from the photoelectron spectra is smaller in 12C180; than in 12C160i, and further reduced in 13,CO$_The fluorescence quantum yields of the A and B states in the heavier ions are consequently closer to unity with respect to the situation in 12CO$. According to the model discussed briefly in section 1, and in detail elsewhere [6], the results presented above indicate -hat a shift of emission intensity occurs from the 2800-2900 A region to the 3000%-5000 A region, due to interelectronic state coupling between A21T, and B 2Zz, but that its extent decreases iu the order 12~160: > 12~180; > 13C160i. It is now of interest to discuss spectroscopic information on interelectronic coupling in CO; and this is done in the following subsection_ 4.1. IntereieHronic
State coupting in CO;
Interelectronic state coupling in a species as small as CO; can manifest itself as spectral perturbations ]16] _Cauyacq et al. [17J have carried out a high resolution study of the E 2Zc - ? 211g(0, 0,O) - (0, 0,O) band of 12C160z. Rotational analysis of this band showed the existence of a small perturbation around J’ = 36.5, previously observed by Bueso-Sanllehi [18], and also, for the first time, a strong perturbation at levels below J’ = 20.5. Fig. 6 gives the scaIed rotational term values ?(N)0.37N(N f 1) as a function of N(N + 1) for the 5 22i (0, 0,O) level. The 0.37 scaling factor was chosen to cIearly reveal spin-splitting anomalies and the perturbation features_ Avoided level crossing between the f rotational levels* of the perturbed E 2Ci state and the unknown perturbing state En occurs in the neighbour-
j
\+
,doo
,
.i” :.:f 2uoo
t4
,
1
N+1)300”
Fig. 6. 2 *Zt state of CO;. Scaled term values T(N) 0.37N(N + 1) as a function ofN(N + 1). Becauseof the scale expansion the 13 rotational constant of the perturbing state appears negative.
hood ofJ’ = 16.5. These f levels are displaced by up to 15 cm-l. The e levels of the 3 2Zi(0, 0,O) state are also perturbed at low J’ values, with maximum displacement of 3 cm-l. The perturbing state Ep lies at about 34620 cm-l above the “x 2111g ground state, i.e. about 20 cm-l above E2ZBi (0, 0,O) (fig_ 6). Avoided level crossing can occur because analysis shows that the E state has a smaller rotational constant than the 522f(O, 0,O) level. Ep is therefore about 6120 crn_-l above the mean enerOv of the x 2TI1,2(0, 0,O) and A 21T,,2(0, 0,O) levels (the spin orbit split’tig of the “A211u state is about 96 cm-l). The low-lying excited states likely to be in the neighbourhood of the E 2Zz(0, 0,O) level are (in linear geome-
levels with parity +(-l#+s-l are e levels,.those w&h parity -(-l)?S-l are f levels. In the % ‘2: state of. CO;, the F1 levels fJ’ = N’ + l/2) are alle, the Fa levels (J’ ‘N’ - l/2) aref. Selection rules forperturbation are AJ= 0,e*e,f*f,e+f.
* Rotational
S. Leachetal./Fluorescencequantumyieldsof isotopicCOf ions
tly) .. . 302, 17r417r250 %- [19], . ..34 1Tr; 17r; 27r”4rIi [20] and .. . 3:; 1:: 14 fill=. In their linear configu-
rations the energiesof the two-quartet states are calculated to be at least 1 eV above the !? 2Zi(0, 0,O) level ]19,20]. The x *Tin state is therefore the best candidate for assigning the En state. The electronic configu rations would in any case favour a first order interaction of B *2: with the x *Ru state rather than a higher order interaction with either the 4Z; or a&, state which differ from E *xi by two or more spin orbitals. Potentjal energy surface caIcuIat&rs of the __.30, 1~: lrz B*Zi and _._30: 1~: 1x2 A*&, states were carried out by Horsley [ 191. These calculations were made with molecular wavefunctions represented by a linear combination of 350 configurations constructed from a molecular orbital basis set. Linear combinations of exponential function orbitals, centered on the atoms, with SCF optimized coefficients, were used toconstruct the molecular orbitals. The x *I$, potential energy surface was calculated to intersect With the 5 *Zi state at the (0, 0,O) level of the latter (fig. 7) thus supporting the assignment of x *II as the perturbing state E . Inrerelectronic state couplin~must involve a vibronic level of the x 211Ustate isoenergetic with the % *LB: (0, 0, 0) level and of appropriate symmetry.
E(eVl
-13-
-15
-17 -
-19 -
-21.
, 1
1
r&Al 2
Fig. 7. Potential energy of the B2ZL and ~zl$ states of COf calculatedas a function of r(C0) distance, The energy scale is relative to the energy of the CO? ion.
119
The expression: WR,]G,(u,,
u*, u3)
where G$ = vi -xi, was used to calculate approximate energy levels of the linear x ?-II,, state in order to determine which combinations of its three vibrational modes can form a suitable vibronic level E, isoenergetic with E2Zi(0, 0,O). The symmetry requirements for interaction impose that E, be an ungerade vibronic state. The vibrational symmetries are respectively vl($), v2(fI,), v3(Zz) and the electronic state symmetry of Ep is I$, SO that the product of representations I’(&,) X I’(ul) X I?&) X I’(+) = f’(Ep) implies u2 f u3 = even for E to have ungerade symmetry. WKen the symme’try requirements are satisfied two cases can arise: (a) The two interacting levels may have the same vibrational angular momentum quantum number I (same vibronic species). (b) The two interacting levels may have AI = f 1. Higher Al values are forbidden. AnaIysis of the x 211,, vibrational levels, although progressing [21], is insufficiently advanced to provide complete precise information about anharmonicities, Renner-Teller and Fermi interactions invohing several vibrational quanta. The calculations were therefore done using the following constants for the “A*lIu stateoff2C’60~:,1 = 1131 cm-l [22],u2=440 cm-l [21],~3 =2731 cm-l 231;~;~ a-3 cm-1 i = [221,x& = -1 cm-l [21],xS3 = -10 cm-l;x& 5 cm-l, XL = -20 cm-r x0 = -10 cm-l. The ?I3were chosen arbitrarivalues ofx!j3, xy2 ,xt3 and’ x23 ly but are of the order and sign expected in CO; “A*II,, from comparison with known values for the B02 x *II,, state [24] and the CO2 % 1Zi state [25]. It should benoted that with ET = 6120 cm-l, not more than two quanta of us or five quanta of v1 can participate in the perturbing level. As is clear from eg. (I), Fermi resonance and coupling due to vibrational angular momentum, in particular the Renner-Teller effects, were not explicitly considered in our En calculations_ Instead it was assumed that these interactions give rise to sets of vibromc levels ofdifferingK(K=I~hCII;I=u2,u2 -2, _._1 or0)
whose energy spread will span the region ofEp = 6120 cm-l if the zero-order calculated level is within 300 cm-l of E,. This is a reasonable limit if the number of v2 quanta is not too high. The following vibronic levels of the daft,, state were thus calculated to lie within 6120 + 300 cm-l and to have the necessary ungerade symmetry (i.e. v2 f v3 = even) for interaction with the E 22z O” level; (0,147 O), (1, 5, I), (0,2,2), (326, O), (2,3>1) and (4,4,0). To achieve this interaction, the (0,14,0), (0, 2,2), (3,6,0) and (4,4,0) levels can form K = 1 (If,) vibronic states, w,hile (1,5, 1) and (2,3, 1) can form K = 0 (Xi) states. .Fig. 6 shows that the E 21Zi (0, 0.0) e and f rotational levels are perturbed differently at low Nvalues. A *Zt vibronic perturbing state (perturbation heterogeneous in I, homogeneous in K) would be expected to affect about equally the e and f levels of a particuIarNvahte of the E21ZL perturbed state since the spin splitting is relatively small at low N values. Marked differences in perturbation of the e and f levels are more characteristic of interaction with a ‘II, vibronic state (perturbation homogeneous in I, heterogeneous in K) whose Q = l/2 and 52 = 312 levels would perturb different N levels. From this we conclude that the four levels (0, 14,0), (0,2,2), (3,6,0) and (4,4,0) are possible perturbing states. The Franck-Condon factors are not likely to be large for the (0,14,0) level. The most probable candidates for En are therefore the D, Renner-Teller vibronic states (eventually modified by Fermi resonance interactions) issuing From the (0,2,2), (3,6,0) and (4,4,0) vibrational levels of x 2ffu. Detailed optical spectroscopic analysis in progress should yield further information on the nature of the observed perturbations. The sparseness of the possible perturbing levels, and the resonance limit type interaction as revealed by the rotational perturbations, make it conceivable that isotopic substitution could markedly affect the interelectronic state perturbation. This is demonstrated not only by the isotopic eft&; of fluorescence quantum yields of the x211U and B Z, states (table l), but also in the spectral perturbations. Work in progress [26] on the analysis of the % 28t - 2 211p0: band of the isotopic species 13C1601 and 12C180$ indicates that, as compared with 12C160;, spectral perturbations are less severe in 12C1gOi and even less so in 13~160$
This behaviour correlates with the quantum yield trends. Further quantitative data on interelectronic state coupling for the isotopic CO; ions requires completion of the high resolution spectral analyses under way-
5. Conclusion In this work we have shown that the photon emission yields of the x2& and % 2Ci states of the CO; ion are markedly isotope sensitivk, whereas the x/E population ratios as determined from 584Aphotoelec tron spectroscopy are identical for the pair of isotopic species 12C1602, 13Ci60,. These results can be interpreted in terms of a model [6] which shows that interelectronic coupling between the E 2Zi and x 2fIu states of CO; can give rise: (a) to a significant shift of photon emission intensity from the spectral region usually considered to be x 2,~: - g 2,“s (2800-2900 li), to that considered to be A 2nd - X2$ (3000-5000 A) ‘; (b) to only a very limited spectral redistribution in the photoelectron spectrum. Our interpretation also requires interelectronic coupling to be isotope sensitive in CO;. The observation that the degree of spectral perturbations in the isotopic ions correlates with the trend of the quantum yields away from unity supports the above interpretation. We conclude, furthermore, that interelectronic interactions explain the much discussed discrepancy in
the apparent population ratio of the x 2ffu and % 22: states of CO; ions, formed by 584 A photoionization of C02, between the results of photoelectron spectroscopy [I,1 1,15] and those of photon emission from the ion states [2-S]_ This discrepancy is implicitly important in a number of physical situations, some of which we will briefly discuss. The first situation concerns the Martian dayglow in which -2B Z,f -X-2 f$andA -2 ff, -X-2 f&emissions of CO; are important features. Airglow models involve
* Direct confmation of a shift in emission intensity from the 2900
A to the 30004000
A region in ‘2C’60~
is given by
recent threshold photoeIectron-photoion coincidence experiments on CO2 In which photon emission was observed at A > 3300 A In coincidence with CO; ions formed in the B%;(O, 0,O) level [30].
interpretation of the intensides of these emissions and this has been done in terms of photoionization, fluorescent scattering and photoelectron impact mechanisms [27-29 J. Interelectronic coupling was ignored in these interpretations. Effects on the results of Martian airglow model calculations, when B *Xix *If,, coupling is specifically taken into account, are discussed elsewhere [3 11. Other cases where interelectronic coupling between the emitting states of CO; should be taken into account concern the determination of the cross sections for formation of the 3 2Xi and x %Iu states of CO; by other than photon impact. These include fast particle collisions with CO2 [32,33] and Penning‘[34] and electron impact [35,36] ionization of CO,. In each of these cases, the ratio of the emission intensities in the 2800-2900 a and 3000-5000 A regions was considered to be directly proportional to the ratio of the g *Xi and ;\‘*lI,, state populations. We believe that intereiectronic coupling makes this invalid, as we have demonstrated above. As a final example where the CO; ion excited state apparent population discrepancy can lead to doubtful interpretations, we mention the work of Strickland and Green [37] on electron impact cross sections for CO?. In this, the population discrepancy, taken at its face value, suggested to these authors that the relative magnitudes of the optical oscillator strength for the E *Xi and x *II,, ionization continua might differ from those of the generalized oscillator strengths. Our results indicate that this is an unnecessary consideration in this particular instance. Acknowledgement We wish to thank D. Gauyacq, J.A. Horsley, J.P. Maier and J.A.R. Samson for making available unpublished results. References [ 11 J.A.R. Samson and J.L. Gardner, I. Geophys. Res. 78 (1973) 3663. [2]
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