3060
J. Phys. Chem. 1984,88, 3060-3069
Electronic Excitation and Quenching of CS Formed in the ArF Laser Photolysis of CS2 G. Dornhiifer, W. Hack,* and W. Langelt Max- Planck-Institut fur Stromungsforschung, 0 - 3 4 0 0 Gottingen, West Germany (Received: June 27, 1983; In Final Form: December 20, 1983)
The exciplex ArF laser photolysis of CS2 at pressures of 0.1-1 mbar and in the presence of up to 200 mbar of Ar or N, was investigated. The emission from two electronic states of CS A‘I2+ and A’II and from C(3lPO)atoms was observed. The time-resolved fluorescenceof CS was studied spectrally. At low pressures the fluorescencein the spectral range of 200-570 nm was assigned to the transitions CS A’lB+(v’= 2, 5) X12+(v”= 5-38). At high pressyes fluorescence appears from vibrational levels other than v’ = 2 or 5 as well as emission from CS A’II is observed, and the band shapes are deformed. The collision-free lifetime of CS A”Z+ was determined to be Q = 15 ns. Quenching rates of CS A’(v’ = 5) for Ar, N2, and CS2are 6 X 9 X loL3,and 50 X 10” cm3/(mol s), respectively. The quenching by Ar and N2 is mainly due to vibrational deactivation. The rotational relaxation of CS A’(v’ = 2) by Ar proceeds with an approximate rate k, = (2 & 1) X lOI3 cm3/(mol s). Based on these results, a discussion of the CS2 photolysis at 193 nm by an ArF laser is given.
-
Introduction Lasers opened up new possibilities in photochemistry for two reasons: (i) The high spectral intensity of the emitted light enables processes which employ the simultaneous or successive absorption of more than one quantum per molecule. (ii) The concentration of the laser intensity in a small wavelength range allows for excitation of molecules to specific, well-defined states. The appearance of the rare gas halide lasers introduced a new technique which makes use of both of these properties of the laser light. During the same laser pulse radicals are generated by photodissociation of a precursor and excited to a higher electronic state. 1-4 The photophysics of free radicals or small reactive molecules like C S is of major interest in this context since they are intermediates in many chemical reactions and they have well-separated energy levels which can be specifically excited. Thus, the induction of nonthermal reactions, which is an important property of laser photochemistry,z can be achieved by exciting the intermediates involved in a reaction system. This work deals ‘with the CS molecule, which is a common intermediate in the ieactions of carbon-sulfur compounds. A scheme for the separation of sulfur isotopes was proposed employing the photodissociation of CS2.6 A shortcoming of this method is, however, that no scavenging reactions for CS are known. The molecule is very unreactive in its electronic ground state7-*and tends to polymerize at the reactor wall rather than to react in the gas phase. The electronic excitation of the C S molecules could improve the separation method if C S in the upper state reacts reasonably fast in the gas phase. Laser-induced reactions can only be studied systematically if extended knowledge of the photophysics is available; i.e., the spectroscopy and the mechanism of laser excitation and quenching in the investigated system should be known. The spectioscopy of C S has been well studied, and a number of singlet and triplet states are reported in the l i t e r a t ~ r e . The ~ three lowest singlet states are well-defined by their spectroscopic data. The ground-state XIC+and the AIII state with an excitation energy of 38 798 cm-I have similar vibration frequencies (V = 1285 cm-l (X’C+)and 1073 cm-I (A’II)) and nearly the same bond length (0.15349 and 0.1574 nm, respectively). The AIII state has a lifetime of 176 ns.I0 The A’IC+ state (T, = 56 505 cm-I) is known only since 1972 by its transitions to the ground state in the vacuum-UV.” Its spectroscopic constants (we = 462.4 cm-’; r = 0.1944 nm) are drastically different from those of X’C+and A’II. No lifetime data have been reported yet. All three singlet states’are bound up to extremely high excitation energies, the dissociation energy of the molecule in the ground state being 59 320 cm-I.’I +Presentaddress: Institut Laue-Langevin, 156 X Centre de Tri, F-38042 Grenoble, France.
The thermal dissociation of CS2 producing C S and S atoms has been studied intensively.I2 Also, the preparation of C S by photodissociation of CS2 is a well-known process.’)
cs,~cs+s
(1)
The ArF laser has turned out to be an efficient radiation source for this phot~reaction.**~*’~ Butler et al. have stated that CS X’C+ is excited by the ArF laser to states which fluoresce in a wide spectral range (170-630 nrn)., Part of.this fluorescence has been assigned to C S A’ (v’ = 5,6).’ An important part of the fluorescence could not be assigned, however. Because no measurements of the decay time were possible, it could not be stated which parts of the fluorescence originated from the same upper electronic state. No data are available on the quenching of the fluorescence of the excited (A’’C+) state of CS, whereas several quenching experiments have been performed for the A’II statelSJ6 and in more detail recently by Simons et al.” Many of the former studies on CS2 ArF laser photolysis dealt with the excess energy distribution to vibrational energy in CS(XIC+)or to electronic excitation of the S atom formed in this process. Two groups have tried to probe the vibrational population in CS(X) directly by tuning a dye laser to the transitions CS A’II(v’) CS XIC+(u” = ~ ’ - l ) . ~ Their 9 ~ ~ result that mainly vibrational levels with v” = 2 up to v” = 5 are excited with a maximum at v” = 3 would enable us to excite C S radicals, formed in the CS, photodissociation, wrth a considerable efficiency. The
-
(1) F. B. Wampler, J. J. Tiee, W. W. Rice, and R. C. Oldenborg, J. Chem. Phys., 41, 1199 (1964). (2) J. E. Butler, W. S. Drozdoski, and J. R. McDonald, Chem. Phys., 50, 413 (1980). (3) C . L. Sam and T. Yardley, Chem. Phys. Lett., 61, 509 (1979). (4) W. Hack and W. Langel, J. Photochern., 21, 105 (1983). (5) A. Ben-Shaul, Y. Haas, K. L. Kompa, and R. D. Levine, Springer Ser. Chem. Phys., IO (1981). (6) T. R. Loree, J. H. Clark, K. B. Butterfield, J. L. Lyman, and R. Engleman, J . Photochem, 10, 359 (1979). (7) E. J. Wright, J. Phys. Chbm., 64, 1648 (1960). (8) J. E. Wollrab and R. L. Rasmussen, J. Chem. Phys., 58,4702 (1973). (9) S. M. Sbchard, Aerospace Report No. TR-007 (4641)-6. (10) S. J. Silvers and C. L. Chiu, J . Chem. Phys., 56, 5663 (1972). (1 1) S. Bell, T.L.Ng, and C. Suggitt, J. Mol. Spectrosc., 44, 267 (1972). (12) F. Rosenkranz and H. Gg. Wagner, Z . Phys. Chem. (Wiesbaden),61, 302 (1968). (1 3) H. Okabe, ‘Photochemistry of Small Molecules”,Wiley-Interscience, New York. 1978. (14) S . C. Yang, A. Freedman, M. Kawasaki, and R. Bersohn, J . Chem. Phys., 72, 4058 (1980). (15) A. J. Hynes and J. H.Brophy, J . Photochem, 9, 145 (1978). (16) A. J. Hynes and J. H. Brophy, Chem. Phys. Lett., 63, 93 (1979). (17) M. N. R. Ashfold. A. M. Ouinton, and J. P. Simons, J. Chem. SOC., Faraday Trans. 2, 76, 905,‘ 915 (1‘980).
0022-3654/84/2088-3060$01.50/00 1984 American Chemical Society
The Journal of Physical Chemistry, Vol. 88, No. 14, 1984 3061
C S Formed in the ArF Laser Photolysis of CS2
I [a u
,
, ,
'? , , , ,:,,
' ' '
';D""
P-blanch
":5-"
75
R-bronch
-
Figure 1. Spectrum of the ArF laser and assignments of the main absorption lines of the transition CS A"C+ X'X'.
results mentioned above2J4s18were correlated to the branching ratio r = [S'D]/[S3P]. If S(lD2) is generated, the highest vibrational level of C S ( X I C + ) which can be populated is u" = 5, whereas C S X 1 C +(u" I 13) is possible for the other channel. The vibrational distribution in CS(X'C+) as found by dye-laser-induced fluorescence is consistent with a high yield of electronically excited S('D2) atoms from reaction 1 (80% according to Yang14). This is in marked disagreement with a direct observation of sulfur atoms18 by vacuum-UV fluorescence, where only 15% S(lD2) were found. In spite of this discrepancy, it is obvious that a major amount of CS2dissociates to ground-state C S which can be excited by a second ArF laser photon. The aim of this work was to observe the states of CS reached by this step and to measure the rates and mechanism of the quenching processes for these states. These results are of interest for selectively induced photoreactions.
Experimental Section The experimental setup has been described earlier in ref 19. The homemade exciplex laser was used on the A r F line at 193 nm with up to 40 mJ per pulse at 2-3 pps. The pulse was slightly shorter than the KrF emission from the same laser and had a 15-ns fwhm. The spectrum is shown in Figure 1. Since the laser beam passed about 1 m through the ambient air, strong absorptions of O2 are also observed (compare ref 2). The laser beam irradiated the gaseous sample in a room-temperature stainless-steel cell (diameter 80 mm) with four windows of fused silica (Suprasil from Heraeus). The entrance window consisted of a lens which focused the beam to the center of the cell. The cell was part of a vacuum line which was operated at pressures from 0.01 to 1000 mbar. In all experiments flowing gas was used to exchange the sample from pulse to pulse. An accumulation of reaction products from S atoms and CS molecules, especially polymers, was therefore avoided, and no significant coating of the reactor walls was observed. The gases were supplied by an MKS 254 flow controller to which pressure heads MKS 220 Baratron (0-10 torr or 0-1000 mbar) were coupled. This enabled the overall pressure of the sample as well as the partial pressures of its components to be kept constant for a long time during the fluorescence measurements. With the same setup the partial pressures of the various species could be varied independently of each other. This was found to be necessary to gain accurate data for Stern-Volmer plots. During all quenching measurements with third gases, the precursor CS2was always present. Since it quenches the fluorescence of CS extremely efficiently, care had to be taken that this process did not influence the results with third gases. The Stern-Volmer plots were obtained at constant partial pressure of CS2and various pressures of the quencher. This procedure gives the correct quenching rate of the third gas. (18) M. C. Addison, R. J. Donovan, and C . Fotakis, Chem. Phys. Lett., 74,58 (1980).
ZUO V":
V"=
IO IC
300
20
15 15
I " "
20
LOO
25
2'5
"
'
'
I
30
'
500
P, , , , , ,
~ [ n m ~.~ , ; ~
, , I
35
"':5
Figure 2. Fluorescence spectrum of CS A"X' X'X' at 0.19 mbar of pure CS2with assignments. The intense emission line at 247.8 nm due to the transition C(31P0 2%) is omitted.
-
--+
Chemicals with the following purities were used: CS2 (Fluka AG, for IR spectroscopy), Ar (Messer Griesheim, 99,9994%), and N2 (Messer Griesheim, 99,9995%). All substances were used without further purification; CS2 was carefully degassed before use. The optical arrangement has been described earlier.19 The components were mounted on a stable four-rod optical bench (Spindler and Hoyer), and thus a good reproducibility of the intensity measurements was obtained. A large spatial separation (about 70 cm) between the fluorescence cell and entrance slit of the monochromator was provided, in contrast to most comparable setups. Thus, the divergence of the detected fluorescence light matches the aperture of the monochromator (THR, Jobin Yvon, f/13.6), and no stray light was observed. The geometry also suppresses the detection of the emission from the entrance window. This is an important factor since even high-quality silica compounds tend to show some emission when irradiated by an ArF laser of sufficient intensity.20 The resolution of the monochromator was selected between 0.03 and 0.3 nm. At wavelength settings above 380 nm glass filters (Schott) prevented UV light, which was not absorbed by the ambient air, from passing through the monochromator in a higher order. The photomultiplier coupled to the exit of the monochromator had a quartz tube (R 955 Hamamatsu) and was cooled to -30 O C by a Peltier element in its housing (Pacific Precision Instruments) in order to reduce the noise level at longer wavelengths. A combination of a transient recorder (R 7912 Tektronix) and a minicomputer PDP 11/04 (Digital Equipment Corp.) was used for recording and evaluating the spectrally and time-resolved fluorescence and for the simulation of spectra. The evaluation of decay times by convolution was described earlier.4J9,21 The range of decay times open to this method depends on the length of the excitation function which has to be convoluted with an exponential decay function. We call the time dependence of the formation of a species of molecules in a given state its excitation function". In cases in which the excitation function has the time dependence of the laser pulse, decay times down to 3 ns can be evaluated. Spectra were recorded by averaging over 10-30 signals from the transient recorder and then taking the average value over a selected time range as relative intensity at the given wavelength. In contrast to working with a boxcar integrator, the shape of the signal can be controlled continuously during the measurement. Moreover, the fully digital processing of the signal eliminates the (19) W. Hack and W. Langel, 11, Nuouo Cimenro Sot. Ital. Pis. E , 63B, 207 (1981). (20) D.J. Wren, Appl. Specfrosc., 34, 627 (1980). (21) W. Hack and W. Langel, Chem. Pys. Lett., 81, 387 (1981).
3062 The Journal of Physical Chemistry, Vol. 88, No. 14, 1984
Dornhofer et al.
I J‘=
‘ \ I I
v’-2 v”-25
J”:
30 2 5 20 ~ - b \ 1‘ I \ #I;$‘ p- branch 25 2
35
\
‘I
30
\
I
. 2 0
5
25 20
25 30
,
~
~
~
h
29 000
28700
Figure 3. Fluorescence spectrum of CS A’IC’ -.X1C’(u’=5+u”=26 lower trace, 0.17 mbar of CS2.
30
~
,
, z5
20
15,
16, ~ 1 2 0 ,
1 1 ’ ’
Figure 4. Fluorescence spectrum of CS A”Ct and u’=2--*u”=10)at 224 nm.
,
,
15
, a
u’:?
v’=K
v’:2
Y
and u’=2-~’’=25)at 345 nm: upper trace, 1 mbar of CS2 in 99 mbar of Ar;
P - branch
”=
:10 R. branch
V.5 Y ” = l l P - branch v = 5 ~ ‘ : l l R - biOnCh
’
-
I lau
300
X1C+(u’=5-.u”=l
1
difficulties which can arise from drift and inexact zero adjustment of gated analogic circuits as used in a boxcar integrator. Results The photolysis of CS2at room temperature by an ArF laser at 193 nm was performed in the gas phase in pure CS2 and in the presence of up to 200 mbar of Ar or N1. An intensive emission in the spectral range from 190 to 570 nm was obtained. The fluorescence spectrum observed immediately with the laser pulse
320
Figure 5. Fluorescence spectrum of CS A”Ct
of CS2 in 99 mbar of Ar.
-
340
[nm]
X 1 x t with 1 mbar
at low pressure of pure CS2is shown in Figure 2. A regular band spectrum with a periodicity of less than 500 cm-l is to be seen in the range 190-320 nm. Between 320 and 380 nm the bands split into a double-peaked structure. At wavelengths longer than 380 nm bands with very low intensity were obtained which become more and more irregular. This emission is due to CS, a photofragment of CS2. The assignment given in Figure 2 indicates that the transition CS A’lX+-X’X+ gives rise to the emission. In order
CS Formed in the ArF Laser Photolysis of CS2
The Journal of Physical Chemistry, Vol. 88, No. 14, 1984 3063
t
to the
Figure 6.
laser pulse). to confirm the assignment and to obtain the rotational structure, fluorescence spectra with higher resolution ( A i = 0.02 nm) were recorded and are shown for two different peaks at 224 and 345 nm in Figure 3 and 4, respectively. Moreover, spectra at higher pressure were observed. Under these conditions the emission spectra change significantly as a comparison between the spectrum in Figure 5 taken at a pressure of 99 mbar of Ar and the one on figure 2 shows. The assignment in Figure 5 demonstrates that further vibrational levels in the A ” C + state are populated by collisions. The rotational distribution is also altered at high pressure as the comparison of the upper and lower traces in Figure 3 shows. The bands recorded at high pressure are red-shaded in agreement with the extended bond length in the upper state. All spectra described so far were, as mentioned above, observed directly with the laser pulse. If the registration of the emission is delayed for 100 ns after the laser pulse (Le. integrated over the time from 100 to 600 ns after the pulse), another well-structured spectrum was found exclusively at higher pressure. This spectrum is shown in Figure 6, in the wavelength range from 240 to 270 nm. The assignment, given in the next section, shows that the spectrum is due to the transition CS AIII XIC+. In order to determine the dynamics of the excited states of CS, the emission as a function of time was recorded at several fixed wavelengths for various pressures of CS2 as well as Ar and N,. Before we describe these measurements of the time dependence in further detail, we present the assignments of the observed spectra.
-
--
TABLE I: Transitions Absorbing the ArF Laser Lines“
X’Z’ (1) (2) (3) (4)
(5)
A’I2’
U” = 5
u“
-4
U” =
5
U” = U” =
3 4
+
5 u’ = 2 V’ = 6 V’ = 0 V’ = 3 V’ =
P branches J” = 10-27 J” =
16-32 J” = 31-40 J” = 38-47 J” = 39-41
R branches J” = J” = J” = J” = J” =
11-30 19-35 33-42 41-50 42-50
“Given in the order due to the rotational quantum numbers J”.
-
X’C+. The 1. Assignments. The Transition CS A“+ extended spectrum of widely spaced bands (Figure 2) is completely assigned to the transition CS A”Z+ X’C’. As shown in Table I, it is mainly the fluorescence from vibrational levels u’ = 2 and 5 that has to be taken into account. The assignment given in Figure 2 shows that the position and the spacing of the bands correspond well to the fluorescence of these two upper vibronic states. The splitting of the bands into the observed double-peaked features can be understood if the spectrum shown in Figure 2 is interpreted as a superposition of the emission of two different vibronic levels. For this interpretation we concentrate on two rovibronic lines which are at the intensity maximum of the respective bands:
-
CS A” C+(u’=S,J’=19) -,X’ C+(u”=n+ 1 ,J”=20) CS A11C+(u’=2, J’=24)
-
X1C+(u”=n,J”=25)
(a)
3064
The Journal of Physical Chemistry, Vol. 88, No. 14, 1984
Dornhofer et al.
TABLE I1 ~~~
n
separation
shift osbd
calcd
4
2.4 69.3 250
(0)
0 69 243
10 25
67.9 247
~
Equation a gives for n = 4 two absorption lines which are nearly at the same position in the spectrum (within 2.4 cm-I), and thus both match those of the A r F laser. In the emission spectra in Figure 4 and in particular in Figure 3, two lines with increasing distance are observed. This line shift AvShlftarises from the anharmonicity of the ground-state vibraiton and from the decrease of the rotational constant with increasing vibrational quantum number. No other shifts between the emission lines have to be taken into account, because the upper rovibronic states are the same for all n; in the lower electronic state only the vibrational quantum number changes but always in the same way (Le. Au” = 1 for all n). We calculate the resulting shift for the vibrational and rotational states using the linear fits for B”and r”in the X’C+ state as given earlier.22 AIylb = (zy1b(U”=4)- Iylb(U”=5)) (?,,b(U”=n) - b,,b(U”=n+l)) = 2 ~ , ~ , (-4n) (b) For the separation of the two lines of eq a the rotational contribution is Atrot = (trot(~”=4,J”=25) - lrOt(~”=5,J”=20)) (Irot(u”=n,J”=25)- tr,,(u”=n+1,J”=20)) = 230ae(4 - n) (c) The sum of both contributions A5 = AF,,b + AI,,, = (~X,W,+ 23Oae)(4 - n) = 11.56(4 - n) cm-’ (d) are comparable with the observed line shifts (cm-I) (Table 11). We conclude that two effects contribute to the splitting of the bands at high values of u”: (i) The anharmonicity of the CS X state leads to a shift of both bands from u’ = 2 to 5 against each other, which is independent of the rotational quantum number. (ii) The decrease of the rotational constant B” in CS X with increasing u” leads to a narrowing of both bands which reduces their overlap. With exact observation of the double-peaked structure presented in Figure 3, one can recognize that the levels u’ = 0, 3, and 6 with high rotational quantum numbers (see Table I) were also populated. The Transition CS AIII-XlC+. The delayed spectrum observed between 240 and 270 nm (Figure 6) can be assigned to the transition CS A’II-X’C’. Emission from a large number of vibrational levels in AIII is seen (u’= 0-13), their energies above the vibronic ground state range from 39 500 to 51 500 cm-I. The bands are narrower than those of A”C+-X’C+ (the rotational constants of the upper A’II state and electronic ground state are not very different), and only transitions with a small change in the vibrational quantum number can be assigned (Au = 0-3), indicating that the Franck-Condon parabola is nearly closed. The bond lengths in the states A’II and X I C t are nearly equal, consistent with the given assignment. The three bands at 265-270 nm mark the beginning of the second half of the spectrum with Au = 0 to -3, which is not reported here. 2. Time Behauior of the Fluorescence. The fluorescence intensity at 342.1 nm with a spectral resolution of AX = 0.2 nm was measured as a function of time at various CS2pressures and with N2 or Ar added. In the runs with Ar and N2 the partial pressure of CS2 was kept constant (1 mbar) (as described in ref 3). The observed lifetimes strongly decreased with increasing pressure. The Stern-Volmer plots gave straight lines (Figure 7). The intercept yields a collision-free lifetime of T ~ ( C A”C+(u’=5)) S = 15 f 2 ns (22) A. Lagerqvist, H. Westerlund, C. V. Wright, and R. F.Barrow, Ark.
Fys., 14, 387 (1958). (23) G . Herzberg, Mol. Spect., Mol. Struct., Prentice-Hall, New York, 1939, Vol. I.
””
25
50
15
p[rnborl
Figure 7. Stern-Volmer plot of the decay rates of CS A’lE+(u’=5)
measured at 342.1 nm.
4 I [a.u I 1 -
h f = 251.0 nrn
1)
o------0
?y---2-c__, 100
200
300
-
A t = 2551nrn
400
500
rlnsl
Figure 8. Time dependence of the fluorescence from CS A’II at 251 nm.
The time behavior of the emission intensity at low pressure (collisionless conditions) was also observed at other wavelengths (at X = 216,256, 320.8,436.5, 455.2,472.9, and 492 nm assigned to a transition from A”Ct(u’=5) and at X = 344.8 nm to a transition from A”C+(u’=2) and at X = 447, 459, and 496.3 nm); in all cases identical decay times were observed. The high error in r0 takes into account that the fwhm of the laser pulse and fluorescence lifetime are not very different. From the slopes of the Stern-Volmer plots the quenching rates are evaluated:
kAr = (6 f 2)
X lOI3 cm3/(mol s)
kNz= (9 f 3)
X
kcs2 = (50 f 15)
lOI3 cm3/(mol s) X
lOI3 cm3/(mol s)
These data refer to the emission at 342.1 nm, i.e. to the depletion of the A”C+(u’=5) state. In contrast, the lifetime of the A’(u’=2) state observed at 344.8 nm decreased only slightly with pressure, and even at Ar pressures up to 200 mbar it was close to the collision-free lifetime (12 ns). At high pressures the fluorescence signals from lower vibrational levels were thus significantly longer than those from u’ = 5. The time dependence of the fluorescence at 251 nm was recorded at different pressures between 21 and 250 mbar. The result at 250 mbar is shown in Figure 8. The time behavior indicates that several transitions contribute to the emission at this wavelength. The time dependence of the emission intensity cannot be evaluated as a single-exponential decay. Cutting off the first 100 ns, we recorded the A’II-XlC’ spectrum. As can be seen in Figure 8, the signal extends over 500 ns even at this high pressure.
CS Formed in the A r F Laser Photolysis of CS2
The Journal of Physical Chemistry, Vol. 88, No. 14, 1984 3065
0
CSJ’I’ Figure 9. Energy level diagram of excitation, vibrational, and electronic deactivation and rotational relaxation of CS A’IC’.
The bands of the A’II-X’C+ overlap so densely that the fluorescence of a specific vibronic state cannot be isolated from the rest of the spectrum. Thus, the signal in Figure 8 can be understood as the sum of pulses from different upper states, as is indicated by the dashed lines. The observation that different vibrational levels of the A’lI state have different time dependences of their population indicates a strong vibrational deactivation in this state. Rotational Deactivation. As mentioned above, the band shape in the fluorescence spectrum A”C+-X’C+ depends on the pressure due to rotational deactivation in the upper vibronic state. To study this effect quantitatively, the rotational lines of the X(d’=25) at 344 nm were recorded with transition A’(v’=2) high spectral resolution at different Ar pressures up to 200 mbar (compare Figure 3). The intensities of the lines were evaluated, and from a standard spectroscopicformulau the relative population of the rotational levels in A’(u’ = 2) was c a l c ~ l a t e d . ~At~ Ar pressures above 35 mbar the observed distribution was very similar to a Boltzmann distribution and could be described by a rotational temperature T,,,. Its values are higher than the ambient temperature T, and decrease with increasing pressure. Although the initial distribution observed at 0.2 mbar (Figures 3 and 4) is different from a Boltzmann distribution, a Boltzmann-like distribution is rapidly attained by collisions. The observation that such high pressure is necessary to obtain total rotation relaxation shows that a considerable number of rotational inelastic collisions per molecule (in the order of 50) is needed for the rotational relaxation of the initial distribution. An estimate of the relaxation rate constant will be given in the Discussion section.
-
Discussion For the photodissociation of CS, a t 193 nm the following scheme is proposed:
CS2 A’B2
-
-
CS2
h t
CSz AIB2
CS X’C+(~”=0-13)
+
S 3PJ AH = -188.9 kJ/mol ( l a )
CS X’C+(U”=O-~)+ S ‘Dz AH = -78.4 kJ/mol ( l b )
(24) G . Dornhofer, Diplomarbeit, Gottingen, 1981.
(The A H values are given for u” = 0 and the initially populated vibronic state of CS2A1B2). The excitation or photodissociation of CS X is performed via
CS X11+(u”15) -% CS A”C+(v’=O-6)
huL
CS X’C+(V’35)
CS X1x+(v’k13)
C 3P
+ s 3P,
2C ’P + S ID2
(2a) (2b) (2c)
The emission and deactivation channels are CS A“C+(v’)
-
CS X’x+(v”)
(3a)
CS A”C+(u’)
5CS XlC+(u”)
(3b)
-huF
CS A”C+(v’)
CS A”C+(v’)
M
CS A’II(u’)
(3c)
CS A”x+(u’-l)
(34
+
M
The CS A’lI formed in collision in reaction 3c undergoes the corresponding emission or deactivation reactions:
CS A’II(u’) CS A’II(u’) CS A’II(0’)
-huF
-% M
CS X’C+(u”)
CS X’x+(v’’)
CS AlII(u’-l)
(4a) (4b)
(4c) The three basic steps, generation of CS X (reaction l ) , excitation of CS X (reaction 2), and quenching of CS A’ and A (reactions 3 and 4) are discussed separately. The energetics of the processes 1-4 are illustrated in Figure 9. 1. Dissociation of CS2 to CS and Energy Distribution in CS. CS2 has a strong absorption between 185 and 230 nm. The resulting upper state correlates with CS X ’ C + and S ‘D,and S ’PJ,respectively.’* The dissociation energy of CS,, D(CS-S), is 35 985 cm-’, corresponding to a threshold for the dissociation at 277.8 nm.I3 CS2 can thus be photolyzed by a single quantum of the ArF lase:. The CS2 A1B2 state has a lifetime of only about 1 ps and a fluorescence quantum yield of less than 10-3.14 Thus, no fluorescence from CS2 was observed.
3044
Dornhofer et al.
The Journal of Physical Chemistry, Vol. 88, No. 14, 1984
2. Excitation of CS. The ground-state C S molecules in an energy distribution produced in reaction 1 can absorb an ArF photon and reach two other singlet states, the A’II and the A ” C + state. The direct excitation for these two states will be discussed in the following sections. 2.1. The Alll State. The transition CS A’II(u’) X’Cf(u”=1-5) can only absorb light of 193-nm wavelength when a great change of the vibrational quantum number (Au = 15) is induced. Because both electronic states, A and X, have nearly the same bond length, the Franck-Condon factor for such transitions is extremely small. Therefore, we could not find any fluorescence from A’II under collisionless conditions. 2.2. The A ” C + State. A large number of rovibronic lines belonging to five overlapping bands of the transition C S A”C+-X’C+ absorb the ArF radiation (Table I). The relative strengths of these five vibronic transitions are given by the Franck-Condon factors, by the matching of laser spectrum and absorption lines, and by the population of the rotational states concerned. The most intensive vibronic transitions are the first two in Table I, because they are closer to the Franck-Condon parabola than the others.11 On the other hand, the transitions u’ = 5 u” = 5 and u’ = 2 u” = 4 show the most favorable rotational spectrum. We thus expect the transitions 1, 2, and 3 in Table I to be strong, populating u’ = 2, 5, and 6. Indeed, the most important part of the fluorescence is assigned to u ’ = 2 and 5. In contrast to the assignments given in ref 2, u’ = 6 shows much less fluorescence than u’ = 2. This could only be distinguished in the present work by assigning the double-peaked structures. By recording the emission spectra with high resolution, we were able to show that triplet or perturbed states were not involved in the observed transitions. Because the bands overlap, no excitation of a single vibronic level in A ” C f is possible with a free-running or even with prism-tuned6 ArF laser. A selective rovibronic excitation affords an exciplex laser which is tuned to a bandwidth of only some wavenumbers.z6 The convolution method to evaluate the fluorescence decay times requires the knowledge of the excitation function for the analyzed fluorescence. Therefore, we had to derive the relevant excitation function of C S A’ (from its formation mechanism) to determine its decay times correctly. The A’ state is excited by the consecutive steps (1) and (2a). The formation per time of the A’ state thus is not proportional to the laser intensity at that time, and in general the excitation function should be different from the laser pulse. We have calculated the excitation function by numerical integration of the following differential equation: d[CS X]/dt = ([CSZlo - [CS X])J,(t)ti - [CS X]J,(t)ez
Besides the C S molecule, S atoms are generated in reaction 1 which could interfere in the reaction system discussed here. It was suggested earlier” that the singlet sulfur should be very rapidly quenched to ground-state sulfur which might recombine to Sz. S2has an absorption which matches the laser pulse and leads to a fluorescing state.z5 This fluorescence could be a sensitive way to detect Sa. Since the assignments for electronic transitions in S2are ambiguous,25we cannot decide from this work whether the absence of this fluorescence signifies that no Sz at all is formed during the laser pulse or whether the lower state of the absorption is too high to be formed here. The C S molecule is produced in the photodissociation of CS2 only in the electronic ground state. For a one-photon absorption process 1 this is evident from energetic reasons. The direct two-photon absorption should lead to fluorescence from the state C S AIIIZanalogous to the vacuum-UV photodissociation of CSz at about half the wavelength of the KrF line.” Because the fluorescence of the CS A’n was only observed at higher pressures and not at low pressure even at very high power density (in a focused beam), we conclude that C S A ’ n was populated by collisions and not be a two-photon dissociation of CS2. In our experiments no indication for a two-photon absorption of CS2was found. In earlier work,z a broad fluorescence in the range of 250-280 nm at 0.02 mbar of CSz was assigned to the transition C S A’II X I C + induced by two-photon absorption of CSz. We observed a similar effect which in our experiments turned out to be fluorescence from the entrance window rather than from the sample itself. I .2. Vibrational State Distribution in C S X‘C’. The initial vibrational energy distribution in C S produced in the ArF exciplex laser photodissociation of CS2 is of some importance for this work, since electronically excited C S A’ is generated by the absorption of the ArF line only by vibrationally excited C S X I C + from u” levels below u” = 5 (see Table I). As mentioned in the Introduction, with two different methods different vibrational distributions for the CS X I C +ground state were obtained. We did not try to measure this value in this work, since we conclude from our observations that this cannot be done in a straightforward manner. The methods applied in earlier work2J4J8seem to be more complicated than assumed. If the photodissociation of CS(XlC’) in high vibrational levels (initially formed in the photodissociation of CSz) contributes significantly to the production of S atoms, the results given there may be systematically in error. As described in detail in the Appendix, we estimate that about 30% of the C S ( X I C + ) is formed in a vibrational state O-v”-5 and can be excited by a second laser photon. 1.3. Rotational Distribution of CS X’C+(v‘’). The bands of the transition CS A’ X are spread out and thus only partially cover the laser line (Figure 1). The rotational distribution in C S X is therefore of some importance for the absorption of the laser line by this transition. Butler et aLz evaluated the rotational distribution in their dye-laser-induced fluorescence spectra and stated that CS X ’ C + from the photodissociation of CS2 is highly rotationally excited. They described the distribution in CS X(u”=l) at a temperature of 950 K. During this work the shape of the fluorescence band C S A’(u’=2) X(v”=25) was simulated as a function of the rotational distribution in A’(v’=2) as generated by the absorption of the laser X(v”=4). For the rotational states line in the band A’(u’=2) in C S X(u”=4) a thermal distribution with a temperature in the range from 400 to 2000 K was assumed. The simulated fluorescence bands had a form similar to that observed in our experiments. The variation of the calculated band shape with the rotational temperature in the C S X(u”=4) state was not significant, and thus a rotational temperature was not obtainable from these simulations.
where T is the measured decay time. The excitation function for the C S A’ state is the product of concentration of ground-state [CS X](t) and laser intensity J L ( t ) . The calculation were performed varying the unknown coefficients q and e2 over some orders of magnitude. The result was that the excitation function is very close proportional to the laser pulse shifted by a few nanoseconds, since CS A’ appears slightly delayed to the intensity maxima. Thus, the laser pulse was used as the excitation function to evaluate the laser-induced fluorescence (LIF) decay time of C S A’. 3. Fluorescence and Deactivation of Excited CS. 3.1. Collisionless Conditions: CS A’- X . The fluorescence of CS A’ as excited in photolysis of CS2 with an ArF laser extends over a large spectral range. Thus, it was speculatedZthat the long wavelength end of the spectrum came from the fluorescence of singlet states unknown until now or from triplet-triplet transitions,
(25) S. M. Suchard and J. E. Melzer, Aerospace Report No. TR-007 (4641)-6.
(26) H. Egger, T.Srinivasan, K.Hohla, H. Scheingraber, C. R. Vidal, H. Pummer, and C. K. Rhodes, App. Phys. Lett., 39, 37 (1981).
-
-
-
+ -
-
-
-
JL(t) is the laser power in einstein/(s cm2), and el and ez denote effective cross sections for dissociation to CSz and excitation of X. The formation of C S A’ is given by C S A’ d[CS A’]/dt = [CS X]JL(t)e* - [CS A ’ ] ( ~ / T o ) + -
C S Formed in the ArF Laser Photolysis of CS2 etc. We have shown that the whole low-pressure fluorescence is to be assigned to the transition A’-X. The large extension of the observed band spectrum is obviously due to the significant difference of the bond lengths in both states leading to a widely opened Franck-Condon parabola. Moreover, by L I F weaker vibronic transitions can be seen than by other methods. Bell et al.” only found the transition A’(v’=2)-X(vff=2-6) and A’(u’=5)-X(u” = 6-10) from u’ = 2 and 5. From Figure 2 we see that we were able to observe bands with Franck-Condon factors which were smaller by 3 orders of magnitude than those of the bands which were observed in their experiments. Our results in the spectrum of C S A’-X should be of further interest to spectroscopists for two reasons: (i) Ground-state vibrational levels up to u’’ = 38 with a vibrational energy of 70% of the dissociation energy were assigned (Figure 2), and their rotational structure was resolved. The vibronic term can thus be determined with high precision. Until now, the vibrational constants and thus the potential of CS X was determined only from the positions of the levels up to u” = 11. l L Even the potential of spectroscopically one of the best studied diatomic C O XLC+, molecules, was derived only from u” = 0-25, i.e. from levels up to 50% of the dissociation energy. The spectroscopy of the C S A’-X transition thus gives the possibility to determine the interaction potential of two atoms in a molecule with unique precision. (ii) The anharmonicity x,w, and the decrease of the rotational constant with increasing vibrational quantum number, a,, are in most cases not directly accessible but are deduced as fit parameters for small decreases of vibrational and rotational transition energies with increasing vibrational quantum numbers. In our case, the shifts of rotational lines of different bands with respect to each other, which lead to the double-peaked structure, can be measured directly. They are only due to x,w, and a,. The collision-free lifetime of the state C S A’ was measured for the first time in this work. The value of 15 ns shows that the transition to the ground state is much stronger than A’II-X’C+ ( T ~= 176 ns). The oscillator strength of A’-X can now be evaluated to be 0.14. The high radiative decay rate k = 1 / =~ 6.7 X lo7 s-l, makes it necessary to add a high pressure of quenching gas if effective scavenging of C S A’ is to be attained. For photoreactions of C S A’ a collision partner has to be used, which only very weakly absorbs the inducing light of the A r F laser. 3.2. Collision-Induced Deactivation of CS A‘. For the laser-excited C S A”C+(u’,J’) three collision-induced deactivation channels (reaction 3b-d) and the rotational relaxation alter the initially populated state distribution. All these processes can be studied by recording the spectrally and time-resolved fluorescence of the excited state. The results obtained for the rotational relaxation will be discussed in the next paragraph, and at the moment we concentrate on the vibrational and electronic deactivation. Reaction 3b-d shortens the lifetimes of vibronic states. The rates have to be added, and that means the measured rate obtained from the Stern-Volmer plot has to be interpreted as
The Journnl of Physical Chemistry, Vol. 88, No. 14, 1984 3067
deactivation (3d) to determine the ratio k3d(Ar)/kAr= aAr.From the spectra in Figure 3 we see qualitatively that at low pressures the band from u’ = 5 is more intense than that from u’ = 2, whereas at high pressure the intensity of the emission from u’= 2 is higher than that from u’ = 5. This shows that a significant part of C S A’(u’=5) is removed by vibrational deactivation. W e assume the mechanism C S A”C+(u?
+M
C S A’’C+(u’-l)
with u’ = 1-5
(3d)
and a rate constant for the vibrational deactivation kut= ~ ’ k ~ = ~ , i.e. stepwise removal of one quantum per effective collision and linear dependence of the rate on the quantum number.28 This model is often used in literat~re,~’ and recent more sophisticated approaches lead only to minor modification^.^^ The increase of intenstiy of the band u’ = 2 u” = 25 compared to u’ = 5 u” = 26 depends on the branching ratio of vibrational to overall deactivation. The branching ratios of electronic to vibrational deactivation can be calculated directly by using the measured ratios of the band intensities and (3d).24 The results of these estimations are
-
aAr
= 0.9
and
-
aN2
= 0.7
That means that about 10% for Ar and 30% for N2 of the observed quenching are due to electronic deactivation either to the A or the X or any other electronic state. (ii) Vibrational Deactivation. The rates of vibrational deactivation given by the product of the branching ratio and the overall deactivation rate are kfd(Ar) = (5 f 2) x ioi3 cm3/(mol s) k3d(N2) = (6 f 3) X 1013cm3/(mol s) (for C S A’(u’=5))
In terms of the average vibrational energy transferred per collision ( A E ) , this result can be described as ( A E ) Y 100 cm-’. The rate of vibrational deactivation is so high that specific reactions of higher vibrational levels, which were successfully demonstrated for other diatomic molecules, seem to be very unlikely for CS A’(u9. The deactivation of CS A’(u’=5) by inert gases differs noticeably from that generally assumed and found for e.g. CF2 A1B2.31 The rates are extremely high, Le. only 1 order of magnitude below collision number, and electronic deactivation to another electronic excited state is affected by inert gases. Electronic deactivation by inert gases usually is very slow because these gases cannot accept a sufficient amount of energy during a quenching collision. In the case of CS, however, the lower electronic states X and A have vibrational states up to some 60000 cm-l, Le. above the excitation energy of A’(u’=5). Therefore, in contrast to the case for other molecules, electronic deactivation of this state to vibrationally excited C S A’Il and X ’ c with an energy transfer of the order of kT, is possible. This vibrational kAr = k3b(Ar) + k3c(Ar) -k k3d(Ar) excitation should, however, be observed in the fluorescence spectrum of CS A’II-X’C+ (Figure 6). The vibrational levels for Ar for example. The relative importance of the three channels of C S A’II with energies close to that of A’(u’=0-5) are A’IIis discussed here. Vibrational deactivation can be identified by (v’=20-23). In the fluorescence spectrum we assigned the the appearance of fluorescence bands from lower vibrational levels transition from A’II(u’=O-l3). We assume that the vibrational in the same electronic state. The population of the other excited deactivation in C S A’II was so fast (according to step 4c) that electronic states was observed by the delayed fluorescence from only levels much lower than the originally excited vibrational levels this intermediate state. Quenching to the ground state is seen are seen since the emission from the A’II state was only observed as a removal of the excited molecules. at high pressure. ( i ) Electronic Deactivation. The significance of the unexpected The intense vibrational deactivation (3d) explains that the electronic deactivation channel (3c) can be described by the branching ratio of electronic to overall deactivation: k 3 c ( ~ r ) / k ~ r . fluorescence pulse from C S A’(u’=2) is not shortened with increasing pressure and that at high pressure the pulse from this A value for k 3 c ( ~ r ) / kcan ~ r be evaluated from the pressure dependence of the ratio of the fluorescence intensity from the originally excited over the collision-induced fluorescence bands if (28) L. Landau and E. Teller, Phys. Z . Sowjetunion, 10, 34 (1938); E. W. the overall deactivation rate and the collision-free lifetime are Montroll and E. Shuler, J . Chem. Phys., 26, 454 (1957). (29) K. F. Freed and D. F. Heller, J . Chem. Phys., 61, 3942 (1974). known2’ The same argument can be applied to vibrational (27) W. Hack and W. Langel, J . Phys. Chem., 87, 3462 (1983).
(30) A. E. de Pristo, J . Chem. Phys., 73, 4329 (1980). (31) G. Dornhofer, W. Hack, and W. Langel, J . Phys. Chem., 87, 3456 (1983).
3068 The Journal of Physical Chemistry, Vol. 88, No. 14, 1984 state is much longer than that from CS A’(v’=5). We give an extensive discussion here since use of this fact was made to evaluate the rotational relaxation rate of CS A’. At low pressure the observed lifetime of CS A’(u’ = 2) is given by the collision-free lifetime of the A’ state, 70 = 15 ns. With increasing pressure the average lifetime of molecules which reach u’ = 2 and fluoresce out of this state is shortened by collisions (down to 3 ns at 200 mbar of Ar). The fluorescence signal from molecules in u’ = 2 which are directly excited by the laser pulse thus becomes shorter than at low pressures. Molecules, however, which reach u’ = 2 from u’ = 5 by stepwise deactivation via D’ = 4 and 3 have already lived sometime in those three higher vibrational levels. Even though their lifetime in u’ = 2 is only e.g. 3 ns, their overall lifetime in the A’ state can even be much longer than 15 ns. The emission from CS A’(v’=2) observed is the sum of the fluorescence of molecules which reach u’ = 2 by direct excitation (2a) and by stepwise vibrational deactivation (3b). The share of the latter molecules increases with increasing pressure. By coincidence, the opposite effects of lifetime shortening by deactivation in u’ = 2 and the apparent increase of lifetime by enhanced deactivation from u’ = 5 cancel over a wide range of pressures. (iii) Rotational Relaxation. The results for the rotational relaxation are given above in a qualitative form (compare Figure 3). In order to make them comparable to literature data on similar systems, a quantitative estimate is made for the rate of the observed relaxation. Since some confusion about rotational inelastic processes seems to exist in literature, we must, however, first define the process observed. A number of studies deal with the effect of collisions on rotationally excited states. Most of them, as revised in ref 32, give cross sections or rates for the collision-induced depopulation of single states. The development of initially nonequilibrium rotational distributions over a large number of states using empirical models33 is hardly ever discussed in literature. We have chosen the following description to deduce a bimolecular rate k, describing the rotational relaxation of initially hot distribution. The intermediate, partially relaxed distributions are fitted by simulated Boltzmann distributions of a temperature (Trot)which is higher than the ambient temperature (T,). This temperature depends on the relaxation time; at short times it is high, at long time, it approaches the ambient temperature. We describe this dependence by a linear differential equation: (1) dTrot(t)/dt = -kr[Arl(Trot(t) - Tal which is integrated to (11) In [(Trot(O) - Ta)/(Trot(t) - Ta)1 = kr[ArIt In this equation, [Ar] is the argon concentration and k, is a bimolecular rate constant which describes the observed relaxation and can be evaluated from the data. TrOt(O)is the rotational temperature for t = 0. In some publications, the value of k, as defined by eq I is set equal to the deactivation rate of a single rotational state:34 k, = kd (111) As was shown in a more detailed i n ~ e s t i g a t i o nthis , ~ ~ is a very crude approximation. Because the rotational relaxation rate depends not only on the cross section of rotational inelastic collisions, as kd does, but also on the magnitude of average energy transfer AE in these collisions kr = f(AE)kd (111’) Equation I11 only holds in the limit of large energy transfer. Under the more reasonable assumption, that the average energy (32) T. Oka, Adv. At. Mol. Phys., 9, 127 (1973). (33) R. A. Covaleskie and C. S. Parmenter, J . Chem. Phys., 69, 1044 (1978). (34) V. Sethuraman, E. Cerjan, and S.A. Rice, J . Phys. Chem., 87,2021 (1983). (35) W . Hack and W . Langel, Max-Planck-Institut fur Stromungsforschung, Report 13, 1983.
Dornhofer et al.
IArl t ([Arl)[rnbar 10.’ SI Figure 10. Determination of k, according to eq 11’.
transferred per collision is in the order of a few rotational quanta, we find values forf(AE) in the order of 0.01. The determination of k, from eq I1 requires one to evaluate the other parameters in this equation from the experimental data. T J t ) is the rotational temperature at time t . Since, for a recorded fluorescence spectrum, the rotational temperature and the relaxation time both depend on the argon concentration, we rewrite (11) to In [(Trot(i) - Ta)/(Trot([Arl) - Tal1 = kr[Arl t([Arl)
(11’)
Trot(i) is the initial rotational temperature, Trot([Ar]) is the rotational temperature observed at a given Ar concentration, and t([Ar]) is the time during which relaxation has taken place at this argon concentration and is set equal to the decay time of the fluorescence from A’(v’=2). k, is evaluated from this equation as follows. Spectra of the X(urr=25) (Figure 3), measured at transition CS A’(u’=2) different argon concentrations, are fitted by simulated fluorescence spectra from Boltzmann distributions in the upper state with various temperatures. The temperature giving the best fit is taken as Trot([Arl). As was layed out above, the vibrational state u’ = 2 is populated from u’ = 5 in a cascade process which runs parallel to the rotational deacfivation. Thus, the intermediate distributions observed in u’ = 2 contain molecules which were initially excited to this level and molecules which were excited to u’ = 5 and afterwards deactivated to v’= 2. The evaluation of Tro,([ArJ) and t([Ar]) thus is onlp approximate and could be in error by two effects: (1) The initial rogtional distribution in u’ = 5 could be different from that in u’ = 2. Vibrational deactivation from u’ = 5 to 2 can therefore influence the observed rotational distribution in u’ = 2, and thus T,,,([Ar]) can be in error. We see, however, from Figure 3 that, by incident, the initial rotational distributions in u’ = 5 and 2 are yery similar. (2) The vibrational deactivation process itself could deform the rotational distribution of molecules deactivated fiom u’ = 5 to 2 in another way than rotational relaxation in the same vibronic state would do, e.g. by channels of coupled vibrational/rotational deactivation. In this case, the t([Ar]) as determined here would no longer be characteristic for the rotational relaxation. This effect is estimated to be of minor importance, since about 50 inelastic collisions per molecule are needed’for rotational relaxation, but only 3 for vibrational deactivation ((1) v’= 5-4; (2) U’ = 4-3; (3) U ’ = 3-2). Ta, the temperature of the bath, will be close to room temperature (300 K) or slightly higher since the laser pulse can heat up the sample. An upper limit of Ta is obviovsly given by the lowest temperature evaluated for an intermediate distribution, which was 350 K a t 200 mbar of Ar. TIot(i)corresponds to an unrelaxed distribution. For it, an upper and a lower limit can be obtained. It has to be higher than the highest intermediate temperature (490 K at 35 mbar of Ar). On the other hand, Trot(i)has to be lower than the temperature of a Boltzmann distribution with the same average rotational energy
-
J. Phys. Chem. 1984,88, 3069-3074 as the initially excited distribution. This average energy can be evaluated from the low-pressure spectrum (see Figure 3); we find Trot(i) IErot/k = 500 K. With these data a plot of In [(Trot(i)- Ta)/(Trot([Ar1)- Ta)] vs. t([Ar])[Ar] as shown in Figure 10 can be obtained. Because of some uncertainties in the determination of the parameters and the interference of vibrational deactivation, we calculate k, with large error bars to be k, = (2 f 1) X l O I 3 cm3/(mol s) This rate is slightly higher than that found for CF2 AlBl.31 It is shown elsewhere35that these relaxation rates are consistent with rotational deactivation rates in the order of 10l5cm3/(mol s) (as usually given in literature) if the average energy transfer per rotational inelastic collision is in the order of a few rotational quanta. Acknowledgment. We are greatly indebted to Prof. Dr. H. Gg. Wagner for his generous support and stimulating interest. Appendix As mentioned in the introduction, there is a discrepancy in the product-state yield of the CS2ArF laser photolysis. CS molecules in vibrational states below u” = 7 are observed by laser-induced fluorescence probing of CS(Xlc+(u’?) whereas from the probing of the S atoms it can be concluded that mainly high vibrational levels of C S ( X I C + ) should be populated. From the photodissociation of CS X(u”) observed in our experiments, we have to qualify this disagreement by the remark that CS X’C+(v’? formed in the photolysis of CS2absorbs laser photons during the same pulse as it is formed.
CS (X’C+(u”)) -% CS(A”C+(u’)) C(,P)
(2a)
3069
-
C atoms. This fluorescence is induced at 193 nm by the absorption 3lPO 2lD in the C atom. Although no attempts were made to give a quantitative estimation of the amount of C atoms present in our system, the emission proves that reaction 2b or 2c takes place in the system. A discrepancy between the results for the branching ratio of reactions l a and lb, obtained by dye-laser-probing of CS29l4and by probing of S ID2 itself,Is can thus arise from two processes: (i) CS X ’ C + in high vibrational levels as initially formed in the dissociation ( l a ) absorbs the ArF laser pulse and is dissociated (2b). Thus, no dye-laser-induced fluorescence of CS X1C+(u”=6) is found and the amount of S ‘D2 formed in step l b is overestimated. (ii) The photodissociation of CS(X lC+u”)(2b) increases the yield of S 3P atoms because the formation of S ‘D atoms in the photodissociation of C S is energetically impossible. The measured ratio of S ‘D atoms over all S atoms formed during the laser pulse18results in an underestimation of the branching ratio in reaction 1. According to this, the following stoichiometric scheme would be consistent with the results of ref 2 and 14 and of ref 18:
--
CS2
0.75(CS X1C+(u”=0-15)
+ S 3PJ)
+ S’DZ) C ,P + S 3PJ
0.25(CS X1C+(~”=0-5)
0.69(CS X’C’(u’35))
-
(la) (1b) (2b)
Yang thus only found 0.06 of 0.75 highly vibrationally excited molecules from reaction l a , since the other 0.69 had been dissociated by rection 2b. Thus, he evaluated a branching ratio [S ‘D]/[S = 25/] = 4. Donovan, who has measured the ratio of electronically excited S atoms from reaction l b to all S atoms formed in reaction 1 as well as in 2b, found
[S ‘D]/([S 3P] + [S ‘D]) = 0.25/(0.75
+ 0.25 + 0.69) = 0.15
+ S(,PJ,’D2)
(2b,c)
These consecutive steps which partially remove the C S radicals were neglected in the literature. Because the absorption differs in intensity for different vibrational levels u” (see Table I), the initial vibrational distribution is distorted. In the photolysis of CS2at 193 nm we observed a strong emission at 247.8 nm due to the transition 3IP0 2% in
-
From this work we conclude that the primary branching ratio of photodissociation of CS2 is r = 0.25(S ‘D2)/0.75(S 3PJ). In the secondary reaction (2b) 70% of the other C S radicals are dissociated so that we can only excite 30% of the initially formed C S to its A’ state. Registry No. CS,, 75-15-0; CS, 2944-05-0; C, 7440-44-0; Ar, 744037-1; NZ, 7727-37-9.
Photocatalytic Decomposition of Aqueous Hydroxylamine Solution over Anatase and Precious MetaVAnatase Y. Oosawa National Chemical Laboratory for Industry, Higashi Yatabe, Tsukuba, Ibaraki 305, Japan (Received: July 7, 1983)
Aqueous hydroxylamine solution is photocatalytically decomposed over anatase, yielding N2,N20,and NH,. H, is also evolved when precious metal/anatase is used. It is confirmed that the charge balance and the material balance hold well throughout the reaction under a few reaction conditions. The Pt loading has a strong effect on the selectivity of the two electron-consumingpathways: H2 evolution and NH, formation. However, it has almost no effect on the total photocatalytic reaction rate. The reaction is also dependent on the pH of the solution and the kind of metal loaded on the anatase. A reaction scheme is proposed on the basis of these results.
Introduction In recent years, numerous photocatalytic reactions of aqueous solution by the use of semiconductor powders have been studied from the standpoint of photochemical energy conversion,’-6 (1) Yoneyama, H.; Koizumi, M.; Tamura, H. Bull. Chem. SOC.Jpn. 1979, 52, 3449.
0022-3654/84/2088-3069$01.50/0
chemical-chemical energy conversion assisted by light organic s y n t h e s i ~ , elucidation ~~’~ of reaction schemes, and SO on, (2) Lehn, J.-M.; Sauvage, J.-P.; Ziessel, R. Nouu. J . Chim. 1980, 4, 623. (3) Kalyanasundaram, K.; Borgarello, E.; Graetzel, M. Helu. Chim. Acta 1981, 64, 362. (4) Borgarello, E.; Kiwi, J.; Graetzel, M.; Pelizetti, E.; Visca, M. J . Am. Chem. SOC.1982, 104, 2996.
0 1984 American Chemical Society
