J. Phys. Chem. 1983, 87, 4799-4804
be regarded as firmly established. Consideration of the scattering tensor within the electromagnetic enhancement model also may provide an explanation of the variation in the relative intensities of some of the modes on changing from 647.1- to 752.5-nm excitation (see Table I). Considering first the al modes, all three diagonal tensor elements a,,,ayy,and azzcontribute to Raman scattering by al modes, and since the relative magnitudes of these elements may differ for different modes, the Raman intensities may respond differently to changes in the relative size of the electromagnetic field components perpendicular and parallel to the surface. On changing from 647.1- to 752.5-nm excitation the perpendicular field component at any point of the surface increases relative to the parallel component, due to the in1 copper with increase in wavelength, and crease in 1 ~ for thus al modes for which axx2, azz2 > ay: are expected to decrease in SER intensity relative to modes for which aXx2, :,yc < aYy2.As far as we are aware there is no information on the magnitude of the components of a for the various pyridine modes, but there are certainly large differences in the diagonal components of a for the a1 modes of pyridine in aqueous solution, as is shown by the difference in the solution a1band depolarization ratios (see Table I), and thus changes in the SER relative intensities of a1 bands with change in excitation wavelength are entirely consistent with the electromagnetic enhancement model. The increase in the perpendicular field relative to the parallel field component with increase in wavelength also accounts for the changes in the relative intensities of the noma1 modes as the wavelength is increased. Thus as Table I shows, the b2 mode at 1150 cm-', which involves only field
4799
components parallel to the surface (assuming flat-on adsorption of pyridine), diminishes considerably in intensity relative to the 1010-cm-' al mode on changing from 647.1to 752.5-nm excitation, while the a2 and bl modes, which involve both parallel and perpendicular surface fields, are less diminished for this wavelength change. In fact, these results are also quite consistent with eq 1 if the factors of Itlo2 and lelR2, which arise from the boundary conditions for fields normal to the surface, are no longer included, to reflect the effects of the field parallel to the surface. In conclusion we present here the first report of surface-enhanced Raman scattering by copper colloids. The results are consistent with the conclusions of earlier work on silver and gold colloids, and thus the copper colloids which show intense SER scattering are shown to be partially aggregated, as is demonstrated in situ by dynamic light scattering. We show that the excitation profile is related to the measured absorption profile, and demonstrate that the increase in lei2 toward longer wavelengths makes an important contribution to the profile. Thus the excitation profile is consistent with the electromagnetic enhancement model of the SER effect. This model is shown also to enable the relative enhancement and intensity vs. wavelength dependence for modes of different symmetry to be interpreted to suggest the adsorbate orientation at the surface. Some preliminary measurements to illustrate the use of SER spectroscopy to investigate competitive adsorption on copper colloids are also presented. Registry No. Cu, 7440-50-8;Ph2S,139-66-2;PhSH, 108-98-5; pyridine, 110-86-1;4,4'-bipyridine, 553-26-4.
Radiationless Transitions in Excited Electronic States of the Benzene Cation in the Gas Phase 0. BraRbart,+E. Castelluccl,t 0. Dujardln, and S. Leach" Laboratolre de Photophysique MoI6culslre du C.N.R.S., Bitiment 213, Universlt6 Parls-Sud, 9 1405 Orsay Caex, France (Received: February 28, 1983)
Emission from electronic excited states of the benzene-& (Bz(h&)and benzene-de (Bz(d6))gas-phase cations wm searched for by using the PIFCO technique in which mass-selected photoion-fluorescence photon coincidences are counted. The maximum fluorescence efficiency was found to be 4 X for benzene ions produced with He I 584-A excitation. From the derived maximum quantum yields and an estimate of the fluorescent decay rate, the nonradiative relaxation raie k,, of electronic excited states of the benzene cation was determined to be greater than 5 X 10l25-l for the C2AzUstate and greater than 8 X 1O'O s-l for the B2Ezgstate. The absence of an observed deuterium effect on the fluorescenceefficiency is consistent wit) these high k, rates. A discussion is given of possible reJaxation pathw_aysof !he excited states; for the C and B states it is shown that relaxation could take place by C B X and C X interelectronic couplings and by isomerization to Dewar benzene and/or benzvalene cations.
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Introduction The evolution and fate of the benzene cation in its excited electronic states has been the subject of much experimental work, discussion, and conjecture. In a recent 'Permanent address: Racah Institute of Physics, Hebrew University of Jerusalem, Israel. f Permanent address: Istituto di Chimica Fisica, Universita di Firenze, Italy. $ Laboratoire associ6 H l'Universit6 Paris-Sud. 0022-3654/83/2087-4799$01.50/0
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publication, Rosenstock et al.' have presented a critical review of our understanding of the fragmentation behavior of electronically excited C6H6+systems. The present work mainly concerns the dynamics of electronically excited states of the gas-phase benzene cation a t energies below the lowest dissociation limit. This dynamics is intimately linked to the question of whether optical emission occurs (1) H. M. Rosenstock, J. Dannacher, and J. F. Liebman, Radiat. Phys. Chem., 20, 7 (1982).
0 1983 American Chemical Society
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The Journal of Physical Chemistty, Vol. 87, No. 24, 1983
Braitbart et al.
200
I50
I58
J,
W v)
w z w
650
188
2 5
W
z
u
0
z
H
5 450
50
s 0 z
0
n
z n 0
250
t Y A
50
50 I100
I288
I460
I640
1820
2000
CHANNEL NUMBER
Figure 1. Coincidence curves for benzene-h, ionized by 564-A light. The electron coincidences give a time-of-flight mass spectrum (lower curve); photon-ion coincidences (upper curve) correspond to counts of real and apparent coincidences between photons In the 16006000-A wavelength range and photoions. The time scale is 2.5 ns/ channel.
from the lowest electronic excited states of the benzene cation. (We will denote the benzene cation as Bz+ in order to distinguish it from the 216 other possible isomers of C&I~+.) Experiments in which excited electronic states of gasphase Bz+ are formed by photoionization? charge transfer? or electron impact4 have failed to show evidence for optical emission from the parent benzene ions. In only one previous experiment was there an attempt to measure the quantum yield & of Bz+ emission. This was done by Eland et a1.,5 who used the PIFCO technique in which mass-selected photoion-fluorescence photon coincidences are counted. The present PIFCO study on Bz+ was undertaken with an apparatus of improved sensitivity. It includes work on the hexadeuteriobenzene cation in an attempt to see whether deuteration could give rise to observable effects on intramolecular fluorescence quenching in Bz+.
Experimental Section The PIFCO experimental setup used here has been described in detail elsewhere.6 Benzene (Bz(h,J or Bz(dJ) molecules are photoionized in a gas jet by using a He1 (584 A; 21.22 eV) lamp source mounted on a vacuum-UV monochromator. The benzene pressure in the gas jet is ca. lo4 torr. Photoions and photoelectrons are accelerated away from the region of formation by an electric field ( N 100 V/cm) toward microchannel plates and a channeltron, respectively. Fluorescence photons emitted from excited species are detected at right angles directly by an EM1 9635 QB photomultiplier whose wavelength sensitivity domain is 1600-6000 A. Photoion-photoelectron coincidences are first recorded and give a time-of-flight (TOF) mass analysis of parent and fragment ions (Figures 1and 2). Photoion-fluorescence photon coincidences are then counted as a function of delay time, with respect to a particular mass-selected ion, (2) W. A. Chupka in 'Chemical Spectroscopy and Photochemistry in the Vacuum Ultraviolet', C. Sandorfy, P. J. Ausloos, and M. B. Robin, Eds., Reidel, Dordrecht, 1974, p 433. (3) B. 0.Jonsson and E. Lindholm, Ark. Fys., 39, 65 (1969). (4) J. P. Maier in "Kinetics of Ion Molecule Reactions", P. Ausloos, Ed., Plenum Press, New York, p 437. (5) J. H.D. Eland, M. Devoret, and S. Leach, Chern. Phys. Lett., 43, 97 (1976). (6)G.Dujardin, S. Leach, and G. Taieb, Chern. Phys., 46,407 (1980).
,
I100
I280
I460
I648
CHANNEL
1828
20a0
NUMBER
Figure 2. Coincidence curves for benzene-d, ionized by 564-A light. The electron coincidences give a time-of-flight mass spectrum (lower curve); photon-ion coincidences (upper curve) correspond to counts of real and apparent coincidences between photons in the 16006000-A wavelength range and photoions. The time scale is 2.5 ns/ channel.
TABLE I: Peak Assignments in TOF Mass Spectra of Benzene-h, and Benzene-d, ion (mass)
peak
Benzene-h
Benzene-d,
together with the total number of photoions. With this technique we distinguish between fluorescence of ions from that of neutral species, and indeed we can correlate emission with a particular ionic species (parent or fragment). The fluorescence efficiency vF is the ratio of the number of fluorescence photons N p emitted to the total number of photoions N I produced over a given time interval: = N p / N p This measurement requires determination of instrumental factors and calibration with N2+ B2Z,+ X22,+ emission which has unit quantum yielda5 In order to convert fluorescence efficiencies qF into quantum yields &Cj), averaged over occupied vibrational levels of a particular emitting electronic state j, we require a knowledge of the branching ratio f j for that state: i$F(j) = v F / f j . The value of f j is generally obtained from photoelectron spectroscopy carried out with the same photoionizing light source as that used in the coincidence experiments. The determination of f j for the benzene cations is discussed later.
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Results Time-of-Flight Mass Spectra. Mass resolution in the TOF mass spectra presented in Figures 1 and 2 is insufficient to resolve ions differing by unit mass. Peaks cy, 0, y, and 6 are considered to have the contributions given in Table I, and include all ions whose appearance potentials are below 21.22 eV as judged from reported photoionization mass spectra at 584 A.' In particular, for the peak ~
~~
___
____
~~~
(7) J. Berkowitz, 'Photoelectron, Photoionization and Photoelectron Spectroscopy",Academic Press, New York, 1979.
Radiationless Transitions in C8He+ photoions produced with 584-A radiation we expect the intensities of C6H5*and C6H4+ to be respectively -35% and 10% of that of CgH6+.' Similar proportions should exist for the corresponding deuterated ions. We note that the relative photoion signals in Figures 1and 2 have not been corrected for mass-dependent ion detectivity variatiom8 Fluorescence Efficiencies. Photoion-fluorescence photon coincidences were counted for 23 h for both Bz(h,j) and Bz(d6). No obvious fluorescence signal can be seen in (delayed) coincidence with the parent ion TOF mass spectrum CY peak (Figures 1 and 2). We assume henceforward that any ion emission counted as true (delayed) coincidence with the CY peak would be due to C6H6+ions. We thus neglect the possibility of emission from the (smaller numbers of) C6H5+and C6H4+ions. We determined the upper limit for true coincidences as follows. The absence of an obvious fluorescence after 23 h of coincidence counting indicates that the quantum yield is very low and that correlatively the fluorescence decay time will be very short. Indeed, it will be shown that the fluorescence decay period is less than -13 ps (see Appendix). This value is much smaller than lifetimes measurable with the PIFCO technique.6 We actually considered a coincidence signal time window of 50 ns, which is twice the rise time of the coincidence signal observed with N2+(B-+X)emission, on the reasonable assumption that, if Bz+ fluorescence exists, it will have decayed significantly in times of the order of 50 ns. The PIFCO signal corresponding to ion flight times greater than that for the Bz' parent ion must be due to false coincidences alone. We therefore divided the PIFCO spectrum into three consecutive segments a, b, c, each of 50-11s duration, of which the middle segment b is centered a t the parent ion flight time and extends through the chosen 50 ns of coincidence signal. Segments a and c are respectively to smaller and greater ion flight times and should each therefore correspond to "false coincidences" i.e., noise. Coincidences were summed for each segment giving values C(a), C(b), and C(c), respectively. The number of possible true coincidences C(t) was considered as being given by C(t) = C(b) - [C(a) + C(c)]/2. The same procedure was then repeated 7 times, with segments a and c being progressively moved to lower and higher ion time flights, respectively, in interval increments of 50 ns. The upper limit for the number of true coincidences C,(t) was taken as the highest value of C(t) obtained with this procedure. The fluorescence efficiencies are given by qF = cm=(t)/zfhvwhere Z is the number of ions measured during the same time interval as the coincidence count, and fhvis the efficiency of detecting the fluorescence photons. The coefficient fhvis determined as described elsewhere; the procedure includes carrying out separate PIFCO experiments on N2before and after each benzene experiment in order to provide a calibration with the known fluorescence efficiency of the N2+B2Z,+-X2Zgf emission. The qFm= values obtained for the benzene cations are given in Table 11. Quantum Yields. In order to calculate the maximum values of the fluorescence quantum yields of the benzene cations we require the branching ratios for forming possible fluorescing states by photoionization of benzene with the He I source. Let us first examine which excited electronic states of Bz' can be formed by He I photoionization. The molecular
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~
(8) S. Leach, G. Dujardin, and G. Taieb, J . Chim. Phys. Phys.-Chim. B i d . 77, 705 (1980).
The Journal of Physical Chemistry, Vol. 87, No. 24, 1983 4801
TABLE 11: He I Excitation of Benzene and Hexadeuteriobenzene Giving Maximum Ion Fluorescence ~ Ion Electronic ~ ~ State , Branching Ratios Efficiencies v f(j), and Maximum Ion Fluorescence Quantum Yields $F, Averaged over Accessible Vibrational Levels of the Ion Excited State(s) ~
Bz+(h,) < 4 x 10-5 0.17
q Emax
f(B2Ezg)
0.08
f(C2A,,)
f(B+C)BFm"(B)a $Fm=(~)b-
$~x(B+c)
0.25 < 2 . 5 x 10-4 5 x 10l2s-l
k,,(B2Ez,) > 8 x 1Olo s-l From the work of Siebrand16on the effects of deuteration on nonradiative rates in aromatic systems having large electronic energy gaps, one expects that deuteration of the benzene ion should lead to a decrease in the ion nonradiative rate by 1or 2 orders of magnitude, and therefore to an increase in the fluorescence efficiency. In Table 11, we report that vF(rnax) is the same in both Bz+(h6)and Bz+(d6),within experimental error. As stated, these are maximum values determined by the absence of a fluorescence signal above the noise. In our experiments, a deuterium effect on the fluorescence efficiency would be expected to be discernible for k, values of Bz+(hJ smaller than 107-10as-'. The absence of an observed deuteration effect is therefore consistent with the high values of k,, that we have determined. We will now discuts the i_mplicationsof the high nonradiative rate for the B and C states of the benzene cation. Radiationless Transitions from the C(z_A,,) State. We first consider the C2Azustate. The C2AzU-X2El,transition is allowed. The corresponding 2A2u-2Elgtransition is observed, with a high quantum yield, in the C6F6+ion6 but, from our results, not in Bz+(h6) or Bz+(d6). We note however that, in Bz+(h6),2Azuis the second, whereas in c$6+ it is the first, excited electronic state. We recall here the Kasha rule17 that in complex molecules fluorescence occurs only from the lowest excited electronic state, due to rapid interconversion from higher states. Although there are a number of exceptions to this rule, it holds well when the electronic energy gap between interconverting states is not high (
