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Fluorescence lifetimes of the single vibrational levels of H2CS1, D2CS, and Cl2CS in the Ā1A2 state
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F.WO&&cE LIFEn& OF THE SING=_ H&S, l&CS, AbjD:Cl,~ m_ =.&*A2 v$I’E
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Fhorcsatncclifetin&s of thesiqle viironicIcvelsof the first excited sin~et~states of H&S. D2CS, and CI,tkS havk b&n mcamrcd under dhsive IIow conditions The single exponentid lifetimes of H&S (J+CS)‘decrease from 140 jq (182 ps) to 72 ps (95 ps) with ilkRa&g viitatiorwl IcvcIsof the A’A, state At the same excitation energy: the lifetime of H&S (~‘e2)-is -. shorter than that of D2CS (AlA,)_ The differena is attriiuted to a tunneIIingeffect in a non-radiativepro&s. The’dekiy of CIzCS @‘A?) is not singIe expon&iaL Asmming a bi-exponent%I decay. &e shorter fluorcscena IiFetimedcueases with th6 ViiGxionaI Ievek. v&k the Ionger one incrraxs. The diffbetween H&S and CIzCS is d&cuss4 in t&-of _ _ non-radiativepnxuscs inciudingdissoaa~on and inteqem crossing
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Intramolecuhu processes have been studied by .the effects of isotop& and substituents on the fiuorekence lifetimes_ The deuterated formeastiring
G3a,-2sow ---_--_x’A2 187x
maldehyde %pecies deca$ about 16’ times more slowly than the hydrogen-species near the A ‘A, [l]_ This rezadt k~explained
origin
_t
by xix& and
Franck~~ond~n factor- effects on a ~sequential coupling of non-radiati% decay of S, states through broadened S, levels to the.c+tiquum_ A same effect is expected for HzCS and D2CS -because the thermochemical dissociation -energy lies below the S, origin f2]_ When. hjdroken atoms _5re substituted by chlorine atoms, the inte&stem cm&i& is enhkced ad inter&on between singlet -r&d &iplet St&&s becorn& appar&t The fluorescence lifetime may not be single exponen- _ tial for Cl;CS if the in&action is large -T Both H&S and Cl$S absorb visible light-fdr the (XI, _n*) transition to A1A2 and the.energetics -
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is quite small, e.g.; dissociation or ~omeriizttion caused by a tunnelling effect.ComparGn of de-~ cay behavior will allow a better understanding of the~chemistryof tetra:atomic molecules. : _,’ ._ :.L.:.
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at 800-9QO°C in a quartz tube [4]_ Roth reagents gave equal LLP spectra of the H,CS ArA, + z ‘At transition D&S was prepared From (CD&S_ Although methyl distdftde gave stronger sign& than dimethyL suhid% (CD&S is cheap as a solvent for
NMR measurements. Commercially avairable Clgs was used as received_ Emission lifetimes were measured uuder an eFFusive flow condition [2]_ The glass tube For the thiorxrbonyl molecufe is introduced through a quick coupler into a 25’. vacuum chamber evacuated by a 6” diffusion pump. During sample Row the pressure of the chamber was between 1O-~-lO-5 Torr_ The efFusive ffow of thiocarbonyl compounds was intercepted at S-10 mm From the blackened nozzle tip (2 mm i-d_) by a pulsed dye faser light From a Molectron DL-IL The bandwidth was reported to be 0.4 cm-‘_ The interception zone was 60 cm away From the pyrolytig zone- Recause of tbermalimtion in the tube, no hot band was detected in the LIF spectrum [2]_ in order to reduce the scattered laser lighr, the fiuorescence Light was coffected through glass cutoff filters, Toshiba Y49 For X,=440-460 nm. Y-51 For 460-500 nm, and O-55 for 500-540 nm. The signal From a photomultiplier (Hamamatsu Photo&s R95S h _ = 850 n-r) with a 10 kf? terminator =as a ccumuiated in a multichannel analyser (05 ps/channel. 1024 channels) with a pre-amplifier For typically = 20000 iaser shots Under our experimental conditions ES] calcuhttion shows that. when the ffuorescen ce lifetime is less than 300 us. an observed fluorescrnce decay reflects a genuine decay_
3. Results 3.1. SVL [A’iz,)
rahriue
rifeiimes
of H,CS
and D&S
SingIe vibrational be? (SVL) fhortzscen~ decay u-as mezsured at various vibrational Ievels of the A’Az state of H&S and D&!3_ The fluorescence intensity is plotted against time as shown in fig_ 2 in which the decay of D_CS (A) is singfe exponential For 5 c I < 350 ps after the laser pulse_ The edge of scattered laser Light appear at t < 5
ps because of the time constant caused by the detection electronics The decay for H,CS was also single exponential_ The SVL ffuorescence lifetimes for DzCS and H&S are listed in table l_ The radiative lifetime of an excited electronic state may be estimated From measuring the osciliator strength of its ground to exited sate transition according to the Strickier-Berg equation_ The reported value of f 2: 4 x IO-'yields g estimated ri@iative lifetime of =140 ps [4]_ our results are in good agreement with this estimated value The lifetimes measured by us decrease gradually with increasing vibrational energies. The decrease in the lifetimes is Iikely to be due to the - slow increase in non-radiative transition rate as discussed previously although the fluorescence quantum yield +s~ has not- been m easured [2]_ The deuterated thioformafdehyde decays 7 1.3 times more slowly than the hydrogen spe&s near the origin This isotope effect is due to. the isotopic. dependence of the non-radiative rates The SVLs containjng no v, (out-of-pIane, bending) or v, (asymmetric HCH bending) have longer lifetimes than those containing more than one u, or pa mode
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By directlaserpumpinginto the 3; 5-z band 650-S nm. phosphorescence (1>15 ms) was observed as described previously. [21_ In this experiment, the time duration was 5 ps/chartnel for the niultichannel analyser. At h, = 596.4 nm cprresponding to the 4: ji-R~ transition, the emission decay consisted of two component& mainly a short-lived one (= 100 ps) with a small contribution of a long-lived one (> 1.5 ms,?--At X, = 504.6 run, corresponding to the 4:s: A-x transition, fluorescence was observed -without a long-lived component_ At 596-4 nm excitation_ the H state was also generated along with the singlet of H&S-at
state, which
gave
the long-lived
phosphorescence
By pumpingintothepuresingletband at 504.9 MI, no phoqjhore&znce was detecied. Intersystem crossing is not an efjicient intram&cular process in _isolated H&S (A). component_
3.2. SYL radiatipe lifetintes of CZ?CS (AwJA,)
-.
wh& the emission intensity is pIotted ag&st timeas sbownin fig-a where.theexcitationtiai;elength corresp&ds to the 4; &% +&ti&n of Cl&IS, the decay ispot -single .tiponential. Stice Clquthief &d Moule [6] repok+ that they -wefe unable- to ob-e any e&si_~~ q&n direct H3Az
Although an SVL lifetime represents some complicated average of the SRVL lifetime& in this paper we assume eq_ (1)’ for s&&city as wih‘be discusse& The parameter 1212 may be very erroneous because I,” is extrapolated from r = 150 ps to t=O_ AII we can safely say is-that 1210 is = I_ Tabie 2 shows the change of lifetimes of short- and long-lived components with .vibrationaI levels of CI?CS (A). The Iow-Iying b, vibrationd levels are reported to fluoresce in unit quantum yield [3J]_ The CI atoms enhance the singlet-triplet interaction instead of promoting the intemai conversion and hence the observed changes in lifetimes are attributable to the changes of -the intersystem crossing rate. The short-lived compo nents compare well with the lifetimes reported by McDonaId and Brus [3] (= 35 ps) although they assumed singIe exponential decays Gas-phase phosphorescen ce has not been observed on direct excitation of the 5-g transition of CI-CS although the CI,CS Be R transition is ret&rkabIy strong. In the region of the A + z origin, the bands from the two systems have about the same strength_ The 3-2 opticaI density reported by MouIe and Subram&tian [S] was 20 x lo-’ and that of the A-2 system 13 x 10q4_ In our experiment, when Cl&S was excited to the (3 veIo@e
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(1)
the various SVL fhrorescence lifetimes are anaIyzed as tabulated in table 2 There is a question about the reliability of a.naIjzing what may be a multiexponential decay as a bi-exponentiaI_ Our experiment was not a SRVL ffuo rescence measurement by virtue of using a wide band optical excitation over a significant portion of the rotational en-
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4. Dkcussion
4-i_ Non-raciiariue frandion
of H,CS
and QCS
using theWmtten~Rabino&ch aRpro&nation ill] : and.the reported ground state.~~_~tional.fiesuen-,: ties [12]_ The value of p is much too low to havenon-radiative ra:es without a : Iine~broadking mechanism in. the ground state. Lirie-brOadeni+ mechanisms &Id be dissociation or isomerization _ because the. thermochemical energy for ‘dissocia-. tion to Hi+CS is- = 14000 cm-’ [7] land for’ isomerization to- HaH is = 15000 cm-’ * which lie below the S, origin 16394 cm-r_ The.foIlowing sequential mechanism is applicable to H-&S pho. -. tochemistry: H&!S(&
u’) -, H,CS(z,
H&S@,
0”) + H, -i- CS, -+ HaH.
Tbe fIuorescence lifetime d ecreases from 140 ps at 6102 nm excitation to 72 ps at 449.5 nm excitation. as is seen from fig- 3_ We reported previously that the Einstein A coefficient explains partly the change of the fIuorescence lifetimes with excitation energy by observing the SVL fhrorescence spectrum [2]_ But fig 3 suggests tbat the SVLS containing v. or V~ have shorter fhrorcsccnce lifetimes than those containing no V~ or v,. Early works of Yeung and Moore [I] have treated the Sr - S, internal conversion as the most important non-radiative process in H&O photochemistry_ In this photochemistry both v, and va are important promoting modes for the internal conversion [9]_ This aIso skxns to be the case for. the photochem istry of H&S because v4 and vs are effective modes (fig 3). -When a non-radiative process occurs intramokcularIy, the fhrorescence lifetime is determined by the relationship
where r0 is the radiative lifetime, and k, represents the non-radiative decay ‘rate due to imramole&I~~ electronic r&&ation1 Because .H,CS belongs to a small moleculecaie and uo intersystem crossing was experimentalIy observed by us and by Bruno and Steer [lo], a plausible candidate for-the non-radiative mechanism is : internal conversion. Fe Ievel density p of the ground state of H&S and D&S is caIcuIat&I to be 6 and 21 per cm:‘;
0”).
~(1) (IIa)
..
~. (IIb)
in process (I)- an initiaIIy prepared vibrational state [b) is coupled ~intramoIecuIarly to a sparse set of ground states ]a); each of which is coupled to the dissociation or isomerization continuum. The non-radiative decay rate k, of jb) to ]a> is given by
where V,b is a coupling matrix element for a promoting mode 1; ob, 0, being frequencies of lb) and In), respectively_ The damping constant f, is associated with the initial and fmaI states and is given by
where u, is a vibrational quantum number if mode I and co), E’r are constants_ According to &is model, the vs or va mode excitation resuhs in short fluor escence lifetimes or larger_-k, values becauseof -variation of Vib by mode I in process
.* Ab i&o
cahlati~ns of *he S,, surface b~‘Tachiia&et aL [13,’suggest that AH = 21000 T-‘_ for the isomcr&~on at the 4-31G IeveI_-This value musi -be. b&esimat& .Jiy-a fact& of. l-4_’ This_ facior 2 obtaikd by c&&&g the cakulata# (I9500 -cm-‘) and rcpoitc&(l4000 cm-‘) valuks forthe~z+CSprocess. .. :. :-- _-, -:.I .z
1
in
(I)_ the case of H&O the rate-determining step is process (I)_ The uuimoXecular dissociation process (Xia) is fast because (a) the ground state H&O is almosx iso-cncrgetic wi*& the molecular products Hz i CO, (b) the top of the barrier between the two is near the S, origin and hence (c) the unimolecular rate constant is large when H&O is excited above the S, origin as revealed by the photodkociation of a ~molecular beam of H&O p4]_ In the case of H&S. ab initio calculations of the S, surface indicate that the potential barrier V, for Hz + CS is 100 kcal/moi ac the 6-31G/CID lcvcl and 10s kcal/mol at the 4-31G level- These values must be overestimated because for H, + CO V0 calculated at the 4-31G lew9 is 111 kcal/moI while it is experimentally evaluated to be =SO kcal/mol [13]_ Even though Vu for H&S is overestimated. the value may not be as low as 50 kcal/mol corresponding to the S, origin of H2CS. A plausible candidate for the non-radiative mechanism in (II) is a tunnelling process_ The significant isotope effect on the flu0 resccnce lifetime suggests that tunnelling is important for process (II). because it primari ly involves the motion of the hydrogen atoms- In other words r, is small in eq_ (1) and depends strongly on energy- The expression for the unimolecular rate constant which incorporates tunnelling is given by Miller [15]_ The isotope effect is expl&xed by his expression but conceming the absolute value of k,, if the barrier height Vu is as high as calculated by Tachibana et al_ j131, k, must be quite small, especially for the case that all the vibrational modes arc active Wait et al_ 1161 suggested that in the early stage of unimolccular dissociation where sufficient energy is available, it is reasonable to assume complete randomization of energy among all vibrational modes while, as reaction proceeds or available energy dimini&& this assumption may not be valid because some specificmodesarefrozen_ They explain this nonRRKM behavior as a change in coupling of the reaction coordinate mode with ether vibrational modes during reaction. In addition to these effects, the ab initio calculation of the 11, t CS mechanism along the intrinsic reaction coordinate (IRC. 0, mode in this case) suggests that the Pi (out-ofplant bending mode) frequency becomes a negative v&e before the molecule reaches the transi-
tion state, that is; the barriw of V~ pcr&nd.ic& to .the IRC becomes an inverted parabola at a relatively low-energy region [13]. The tunnelling rate increases at this region. -~
4.2. Non-radiafiue lrrursition of Cr,cS life&es of C&S (4) are not The flu0 &ce single ex&ncntial_ They do’ not ~exhibit a monotonic decrease in fI uorescencc lifetimes with changing vibrational energy_ We rationalize these results as follows. The tunnelling rate is quite small because of the heavy mass of the Cl atoms. In addition, the tht%modynamical value for dissociation is above the S, origin_ The dissociation has no contribution the non-radiative of C12CS_Thenon-exponeutialdecaybehaviorisattributable to the coupling between S, and T1 (or G)_ The Douglas effect for NO, and Sq has been explained by the coupling bet&en St and S, (171. With increasing vibrational levels of SO2 and NO,, the s, values become smaller and those of 7s become larger while this is not the case for Cl&S, i-e_ rL becomes large and 7s small. Similar behavior is observed for the dicarbony4~ that is. -7, values become small with increasing excitation energy- The singlet states of these dicarbonyls interact with the triplet states_ We assume that the S,--T, coupling is important for Cl&S because of a small energy gap between S, and T1 (AE= = 1224 cm-‘) and a lax-&espin-orbit mat.+ clement (TJ= 375 cm-‘) [SJ. AE& and q are- three times smaller and lower than for H&O, respectively_ C&S seems to belong to an intermediate molecule case because the Tt state formed _by direct excitation does not phosphoresce but decays by a non-dissociative int&nolccul& process-~ If so, it would bc reasonable to assume a weak coupling model for intermediate molecuIes_ Suppose that each vibrational level of S, interacts weakly with a single Tf level_ Here the double dagger reminds us that the TX level, lying in the energy region of S,, is vibration&y excited A&&ing to Avouris et & flS]. the moleculti eigenfunction corresponding to such a pair car-be written as ’
-.
.
#c= V/A-E, zind ti=~-In this model the excited state emission is a superposition of the radiative decays of the molecular eigenstate I&) and &>. After pulse excitation at zero pressure we have that the emission -intensity wiII fall off accoiditig to with
7s = l/[
(1- p)kS + kZ].
rL = I/(&+
and Z>Zz
kL).
(4)
= p/(1 - p)_
in table 2, the values of rs decrease with increasing photon energy wt, while the values of rr increase_ This fact indicates that the coupling parameter p decreases with oL. The decrease of the coupling parameter may be explained by the change of Franck-Condon factors between the singlet and triplet states with increasing viiraAs
tional
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..-.
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Ackno&dgement The authors wish to thank T_ Suzuki for his help in the experiment and Drs_ M. Koizumi and A. Tachibana for sending us the results of their IRC analysis prior to publication. This work was partly supported by a Grant-in-Aid of the Ministry of Education, Science and Culture.
_Referen&
.-
m_.~
~~ ~--
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_
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.. 111ES
Yeuag and C-B. I&ore,
J. Chk_
Phys. -5s (r97$
398s.
[2] U KawasakJ. K Ka&aui, Y. Ogawa and i . Sate. Chem. Phya 74 (1983) 83.
13)J-R. McDonald z&d L_EBrus.
Chem Phya Let& 16. (1972).687. [4] R-H_ Judge and G-W. King. Can J_ Phy+. 53 (1975) k27; RK Judge and G-W: King. J_ _MoL Spectry_ 74 (1979) 175. [5] M Kawasaki. K Kasatani. S. Tar&m&i. H. Sate and Y_ Fujimura. J. Chem_ Phys. 78 (1983) 7146. 161DJ_ Clouthier and DC Moule, J_ &4oL Speetry_87 (1981). 471.
[7] RP. Steer.Rev_Chem.Intermediates 4 (1981)1; DJ_ CJoufhier. PA Ha&e& AR Knight and R.P_ Sker. J. Photochem_17 (1981) 319. [S] DD. Moule and C-R Sub mmaniau. J. MoL Spauy. 48 (1973) 336. (91 S-II_Lin. Proc Roy. Sot A352 0976) 57. [LO] AE Bruno aad P.R. Steer. J_Chem Phyr 78 (1983) 6660. [II] P_J_Robinson and KA_ Holbrook inr UaimokcuJar reactions (WiJey-Jncerxience, New York. 1972) eq. (S-38). [12] P-H Turner. I- HaJoneu and LM MiBs. J_ MoL Specuy_ 88 (1981) 402_ [13] A Taehiiana. I_ Ohazaki. M Koizumi. K HOI-Jand T_ Yamabe. to be published_ [14] P_ Ho. D_L Barnford. RJ. Buss. Y-T_ Lee and C_B_ Ivloo~ J. Chem_ Phys_ 76 (1982) 3630. [IS] W-H -Xi&r. J_ Am_ C&m_ Sot 101 (1979) 6.910.. 1151BA Waite. SK_ Gray and W-H Miller. J. Chem Phya 78 (1983) 259_ [17] EKC. Lee and G-J- Loper. ix Radiationless traaaitious, ed. SK Liu (Academic Press, New York, 1980) p_ 1. [18] P. Ayourk W_M Gdbert and MA El-Sayed, Chm Rev77 (1977) 793.
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F.WO&&cE LIFEn& OF THE SING=_ H&S, l&CS, AbjD:Cl,~ m_ =.&*A2 v$I’E
V@XRATIONiLi m _. ;
KAWASAKI,-KazuoKASATANIandHiroyasuSATO
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Rcainxi 30 May 1984;in finalform F October 19&a
_
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:-
Fhorcsatncclifetin&s of thesiqle viironicIcvelsof the first excited sin~et~states of H&S. D2CS, and CI,tkS havk b&n mcamrcd under dhsive IIow conditions The single exponentid lifetimes of H&S (J+CS)‘decrease from 140 jq (182 ps) to 72 ps (95 ps) with ilkRa&g viitatiorwl IcvcIsof the A’A, state At the same excitation energy: the lifetime of H&S (~‘e2)-is -. shorter than that of D2CS (AlA,)_ The differena is attriiuted to a tunneIIingeffect in a non-radiativepro&s. The’dekiy of CIzCS @‘A?) is not singIe expon&iaL Asmming a bi-exponent%I decay. &e shorter fluorcscena IiFetimedcueases with th6 ViiGxionaI Ievek. v&k the Ionger one incrraxs. The diffbetween H&S and CIzCS is d&cuss4 in t&-of _ _ non-radiativepnxuscs inciudingdissoaa~on and inteqem crossing
~~‘&3&2iE-
1, lrmoduction
-..
_-_~;~~~---7no
Intramolecuhu processes have been studied by .the effects of isotop& and substituents on the fiuorekence lifetimes_ The deuterated formeastiring
G3a,-2sow ---_--_x’A2 187x
maldehyde %pecies deca$ about 16’ times more slowly than the hydrogen-species near the A ‘A, [l]_ This rezadt k~explained
origin
_t
by xix& and
Franck~~ond~n factor- effects on a ~sequential coupling of non-radiati% decay of S, states through broadened S, levels to the.c+tiquum_ A same effect is expected for HzCS and D2CS -because the thermochemical dissociation -energy lies below the S, origin f2]_ When. hjdroken atoms _5re substituted by chlorine atoms, the inte&stem cm&i& is enhkced ad inter&on between singlet -r&d &iplet St&&s becorn& appar&t The fluorescence lifetime may not be single exponen- _ tial for Cl;CS if the in&action is large -T Both H&S and Cl$S absorb visible light-fdr the (XI, _n*) transition to A1A2 and the.energetics -
&
s&&r,
&_
&e. mole&&r
.
163% ?i3A2 1.cso7-
Hpcs
- lCOo0
%cs
I’&
IX&i&& &en -if k&
acs;ct -moo ~___--a2 -cs-i9soo
llbS2
: ,. -.
z
Fig 1. Elatronicensteer p&
-qcs - _-
.~~
levelsand diskaa- bon limits zfter
is quite small, e.g.; dissociation or ~omeriizttion caused by a tunnelling effect.ComparGn of de-~ cay behavior will allow a better understanding of the~chemistryof tetra:atomic molecules. : _,’ ._ :.L.:.
T.
1:
.-..
_- _
-..- T _
--_
are
~radiati~rateofth~moIeculesissosmall(~~10~4 s)’ [2,3] that-the tioxGradi&i+e~ rrite k;. do affect si&ificantl~. tlie fluo~tiizfi
g
&&&a&n
energi& and th? ele-ctronic extitation ~e@ies cl~ylocatedasshowniq_fig-i_‘Espedall_v,the=
x ‘A7
aan2
7
-~-
&Experimi?ntal---
;
-.
--I.
-*
-_ --:._..:-~~
i_
:
: ,-.:;
~: -. .y f -: --, ! H2CS was g&er+xi~‘ by pyioly& Of: methyl $isuifide (CH,!T&.or +ethyIene.suEde @H&S
()3O1_0104/85/$03.30 a -vie; Sckxe F%+ishers B.G. -(North-HdllandPhysicsPublis~gDivision) .~ ~_.,~ __-,
-: _:
’ .- ‘_,_ I~
.-‘:
y__
: .-_... __
at 800-9QO°C in a quartz tube [4]_ Roth reagents gave equal LLP spectra of the H,CS ArA, + z ‘At transition D&S was prepared From (CD&S_ Although methyl distdftde gave stronger sign& than dimethyL suhid% (CD&S is cheap as a solvent for
NMR measurements. Commercially avairable Clgs was used as received_ Emission lifetimes were measured uuder an eFFusive flow condition [2]_ The glass tube For the thiorxrbonyl molecufe is introduced through a quick coupler into a 25’. vacuum chamber evacuated by a 6” diffusion pump. During sample Row the pressure of the chamber was between 1O-~-lO-5 Torr_ The efFusive ffow of thiocarbonyl compounds was intercepted at S-10 mm From the blackened nozzle tip (2 mm i-d_) by a pulsed dye faser light From a Molectron DL-IL The bandwidth was reported to be 0.4 cm-‘_ The interception zone was 60 cm away From the pyrolytig zone- Recause of tbermalimtion in the tube, no hot band was detected in the LIF spectrum [2]_ in order to reduce the scattered laser lighr, the fiuorescence Light was coffected through glass cutoff filters, Toshiba Y49 For X,=440-460 nm. Y-51 For 460-500 nm, and O-55 for 500-540 nm. The signal From a photomultiplier (Hamamatsu Photo&s R95S h _ = 850 n-r) with a 10 kf? terminator =as a ccumuiated in a multichannel analyser (05 ps/channel. 1024 channels) with a pre-amplifier For typically = 20000 iaser shots Under our experimental conditions ES] calcuhttion shows that. when the ffuorescen ce lifetime is less than 300 us. an observed fluorescrnce decay reflects a genuine decay_
3. Results 3.1. SVL [A’iz,)
rahriue
rifeiimes
of H,CS
and D&S
SingIe vibrational be? (SVL) fhortzscen~ decay u-as mezsured at various vibrational Ievels of the A’Az state of H&S and D&!3_ The fluorescence intensity is plotted against time as shown in fig_ 2 in which the decay of D_CS (A) is singfe exponential For 5 c I < 350 ps after the laser pulse_ The edge of scattered laser Light appear at t < 5
ps because of the time constant caused by the detection electronics The decay for H,CS was also single exponential_ The SVL ffuorescence lifetimes for DzCS and H&S are listed in table l_ The radiative lifetime of an excited electronic state may be estimated From measuring the osciliator strength of its ground to exited sate transition according to the Strickier-Berg equation_ The reported value of f 2: 4 x IO-'yields g estimated ri@iative lifetime of =140 ps [4]_ our results are in good agreement with this estimated value The lifetimes measured by us decrease gradually with increasing vibrational energies. The decrease in the lifetimes is Iikely to be due to the - slow increase in non-radiative transition rate as discussed previously although the fluorescence quantum yield +s~ has not- been m easured [2]_ The deuterated thioformafdehyde decays 7 1.3 times more slowly than the hydrogen spe&s near the origin This isotope effect is due to. the isotopic. dependence of the non-radiative rates The SVLs containjng no v, (out-of-pIane, bending) or v, (asymmetric HCH bending) have longer lifetimes than those containing more than one u, or pa mode
14ce3 9616
..
4’5’ 2’3’4’6’
&G&8 75
lllfl 105
200 .~
3’4’6I 1’4’
82 -
126.
3* 4’6’
125*2
I30
126
3’. 3’4’6’ 324’
150 103 82
1% 109
3’43 45 213’4’ 33
76
regarded
tive transitions_
2oLl
.
94c4 68 69.
127
2’4’s1
134 1.50
3’5’6’ 3251
83&l
124 135 99
3’4’5’ 7’3’4’51 334%5l
72
in fig_ 3_ The v, and v, modes may be as the promoting
5%’ 3’5’ 3x4’5’ 1’2’
9856 101 106+3
3’4’6’ 5’ 4’6’
as shown
:.
121 182
modes
for non-radia-
--.:-
_-_ .-
124 152 109
93 115 117 113 95 -
:
By directlaserpumpinginto the 3; 5-z band 650-S nm. phosphorescence (1>15 ms) was observed as described previously. [21_ In this experiment, the time duration was 5 ps/chartnel for the niultichannel analyser. At h, = 596.4 nm cprresponding to the 4: ji-R~ transition, the emission decay consisted of two component& mainly a short-lived one (= 100 ps) with a small contribution of a long-lived one (> 1.5 ms,?--At X, = 504.6 run, corresponding to the 4:s: A-x transition, fluorescence was observed -without a long-lived component_ At 596-4 nm excitation_ the H state was also generated along with the singlet of H&S-at
state, which
gave
the long-lived
phosphorescence
By pumpingintothepuresingletband at 504.9 MI, no phoqjhore&znce was detecied. Intersystem crossing is not an efjicient intram&cular process in _isolated H&S (A). component_
3.2. SYL radiatipe lifetintes of CZ?CS (AwJA,)
-.
wh& the emission intensity is pIotted ag&st timeas sbownin fig-a where.theexcitationtiai;elength corresp&ds to the 4; &% +&ti&n of Cl&IS, the decay ispot -single .tiponential. Stice Clquthief &d Moule [6] repok+ that they -wefe unable- to ob-e any e&si_~~ q&n direct H3Az
Although an SVL lifetime represents some complicated average of the SRVL lifetime& in this paper we assume eq_ (1)’ for s&&city as wih‘be discusse& The parameter 1212 may be very erroneous because I,” is extrapolated from r = 150 ps to t=O_ AII we can safely say is-that 1210 is = I_ Tabie 2 shows the change of lifetimes of short- and long-lived components with .vibrationaI levels of CI?CS (A). The Iow-Iying b, vibrationd levels are reported to fluoresce in unit quantum yield [3J]_ The CI atoms enhance the singlet-triplet interaction instead of promoting the intemai conversion and hence the observed changes in lifetimes are attributable to the changes of -the intersystem crossing rate. The short-lived compo nents compare well with the lifetimes reported by McDonaId and Brus [3] (= 35 ps) although they assumed singIe exponential decays Gas-phase phosphorescen ce has not been observed on direct excitation of the 5-g transition of CI-CS although the CI,CS Be R transition is ret&rkabIy strong. In the region of the A + z origin, the bands from the two systems have about the same strength_ The 3-2 opticaI density reported by MouIe and Subram&tian [S] was 20 x lo-’ and that of the A-2 system 13 x 10q4_ In our experiment, when Cl&S was excited to the (3 veIo@e
..a
+
*-
ji: excitation,
n;--_&_
--7-A
-----
TiMEIps
the
long-iived
phosphorescence Assuming tensity expression.
component
is
a hi-exponential
f(t)=I,“exp(-r/TL)tJSOexp(--1/75).
not
in-
(1)
the various SVL fhrorescence lifetimes are anaIyzed as tabulated in table 2 There is a question about the reliability of a.naIjzing what may be a multiexponential decay as a bi-exponentiaI_ Our experiment was not a SRVL ffuo rescence measurement by virtue of using a wide band optical excitation over a significant portion of the rotational en-
x b-1 534-4 52&I 52X6
r&m-1)
Assigmnalt
l
521-o
18713 maos 19099 1919s 19209
Qz 43 4’6’ 2’4’ 2’4’
5180 5143 51L6 5095 503-7 501.7
19305 19444 I9508 I%27 I9853 19932
4J 2’3’4’ 3?4’ 114’ 1’3’4’ 1’4= or 2’3’4’
501.6 495.6 494-4
19936 2Os78
520_9
4%5 4912
20358
1’314’
1’2’3’4’
’
Lifctilnc(ps)
IL/IS
l00g
sbon
71 92 96~16 loo
25 22 12&t 30
15
114 122 xx3
1.1
axi2 121 113
23 22 19 26&2 22,4 11 23
113 128 121
23 I1 SO
1.8
153 136
i2 12
SW&l
1.7
1.6+05 1.4
-
4;)
and (Z, s4;)
Iev&
af x& = 562.1 -and 554-4
pm. respectively, no emission was observed. after. an ‘accumulation of 30000 Iaser shots- with R-60 cut-off filters_ We tested &possibility of intersystern crossing after excitation to a sir&et- state. At the-singlet excitation~(X& = Sol_9 run Ali4:) the short decay was observed after as few as 1000 laser. shots with a Y-55 filter but no emission was detected with an R-60 f&r with 30000 laser shots. The quantum’ yield of phosphorescence must be quite low.
4. Dkcussion
4-i_ Non-raciiariue frandion
of H,CS
and QCS
using theWmtten~Rabino&ch aRpro&nation ill] : and.the reported ground state.~~_~tional.fiesuen-,: ties [12]_ The value of p is much too low to havenon-radiative ra:es without a : Iine~broadking mechanism in. the ground state. Lirie-brOadeni+ mechanisms &Id be dissociation or isomerization _ because the. thermochemical energy for ‘dissocia-. tion to Hi+CS is- = 14000 cm-’ [7] land for’ isomerization to- HaH is = 15000 cm-’ * which lie below the S, origin 16394 cm-r_ The.foIlowing sequential mechanism is applicable to H-&S pho. -. tochemistry: H&!S(&
u’) -, H,CS(z,
H&S@,
0”) + H, -i- CS, -+ HaH.
Tbe fIuorescence lifetime d ecreases from 140 ps at 6102 nm excitation to 72 ps at 449.5 nm excitation. as is seen from fig- 3_ We reported previously that the Einstein A coefficient explains partly the change of the fIuorescence lifetimes with excitation energy by observing the SVL fhrorescence spectrum [2]_ But fig 3 suggests tbat the SVLS containing v. or V~ have shorter fhrorcsccnce lifetimes than those containing no V~ or v,. Early works of Yeung and Moore [I] have treated the Sr - S, internal conversion as the most important non-radiative process in H&O photochemistry_ In this photochemistry both v, and va are important promoting modes for the internal conversion [9]_ This aIso skxns to be the case for. the photochem istry of H&S because v4 and vs are effective modes (fig 3). -When a non-radiative process occurs intramokcularIy, the fhrorescence lifetime is determined by the relationship
where r0 is the radiative lifetime, and k, represents the non-radiative decay ‘rate due to imramole&I~~ electronic r&&ation1 Because .H,CS belongs to a small moleculecaie and uo intersystem crossing was experimentalIy observed by us and by Bruno and Steer [lo], a plausible candidate for-the non-radiative mechanism is : internal conversion. Fe Ievel density p of the ground state of H&S and D&S is caIcuIat&I to be 6 and 21 per cm:‘;
0”).
~(1) (IIa)
..
~. (IIb)
in process (I)- an initiaIIy prepared vibrational state [b) is coupled ~intramoIecuIarly to a sparse set of ground states ]a); each of which is coupled to the dissociation or isomerization continuum. The non-radiative decay rate k, of jb) to ]a> is given by
where V,b is a coupling matrix element for a promoting mode 1; ob, 0, being frequencies of lb) and In), respectively_ The damping constant f, is associated with the initial and fmaI states and is given by
where u, is a vibrational quantum number if mode I and co), E’r are constants_ According to &is model, the vs or va mode excitation resuhs in short fluor escence lifetimes or larger_-k, values becauseof -variation of Vib by mode I in process
.* Ab i&o
cahlati~ns of *he S,, surface b~‘Tachiia&et aL [13,’suggest that AH = 21000 T-‘_ for the isomcr&~on at the 4-31G IeveI_-This value musi -be. b&esimat& .Jiy-a fact& of. l-4_’ This_ facior 2 obtaikd by c&&&g the cakulata# (I9500 -cm-‘) and rcpoitc&(l4000 cm-‘) valuks forthe~z+CSprocess. .. :. :-- _-, -:.I .z
1
in
(I)_ the case of H&O the rate-determining step is process (I)_ The uuimoXecular dissociation process (Xia) is fast because (a) the ground state H&O is almosx iso-cncrgetic wi*& the molecular products Hz i CO, (b) the top of the barrier between the two is near the S, origin and hence (c) the unimolecular rate constant is large when H&O is excited above the S, origin as revealed by the photodkociation of a ~molecular beam of H&O p4]_ In the case of H&S. ab initio calculations of the S, surface indicate that the potential barrier V, for Hz + CS is 100 kcal/moi ac the 6-31G/CID lcvcl and 10s kcal/mol at the 4-31G level- These values must be overestimated because for H, + CO V0 calculated at the 4-31G lew9 is 111 kcal/moI while it is experimentally evaluated to be =SO kcal/mol [13]_ Even though Vu for H&S is overestimated. the value may not be as low as 50 kcal/mol corresponding to the S, origin of H2CS. A plausible candidate for the non-radiative mechanism in (II) is a tunnelling process_ The significant isotope effect on the flu0 resccnce lifetime suggests that tunnelling is important for process (II). because it primari ly involves the motion of the hydrogen atoms- In other words r, is small in eq_ (1) and depends strongly on energy- The expression for the unimolecular rate constant which incorporates tunnelling is given by Miller [15]_ The isotope effect is expl&xed by his expression but conceming the absolute value of k,, if the barrier height Vu is as high as calculated by Tachibana et al_ j131, k, must be quite small, especially for the case that all the vibrational modes arc active Wait et al_ 1161 suggested that in the early stage of unimolccular dissociation where sufficient energy is available, it is reasonable to assume complete randomization of energy among all vibrational modes while, as reaction proceeds or available energy dimini&& this assumption may not be valid because some specificmodesarefrozen_ They explain this nonRRKM behavior as a change in coupling of the reaction coordinate mode with ether vibrational modes during reaction. In addition to these effects, the ab initio calculation of the 11, t CS mechanism along the intrinsic reaction coordinate (IRC. 0, mode in this case) suggests that the Pi (out-ofplant bending mode) frequency becomes a negative v&e before the molecule reaches the transi-
tion state, that is; the barriw of V~ pcr&nd.ic& to .the IRC becomes an inverted parabola at a relatively low-energy region [13]. The tunnelling rate increases at this region. -~
4.2. Non-radiafiue lrrursition of Cr,cS life&es of C&S (4) are not The flu0 &ce single ex&ncntial_ They do’ not ~exhibit a monotonic decrease in fI uorescencc lifetimes with changing vibrational energy_ We rationalize these results as follows. The tunnelling rate is quite small because of the heavy mass of the Cl atoms. In addition, the tht%modynamical value for dissociation is above the S, origin_ The dissociation has no contribution the non-radiative of C12CS_Thenon-exponeutialdecaybehaviorisattributable to the coupling between S, and T1 (or G)_ The Douglas effect for NO, and Sq has been explained by the coupling bet&en St and S, (171. With increasing vibrational levels of SO2 and NO,, the s, values become smaller and those of 7s become larger while this is not the case for Cl&S, i-e_ rL becomes large and 7s small. Similar behavior is observed for the dicarbony4~ that is. -7, values become small with increasing excitation energy- The singlet states of these dicarbonyls interact with the triplet states_ We assume that the S,--T, coupling is important for Cl&S because of a small energy gap between S, and T1 (AE= = 1224 cm-‘) and a lax-&espin-orbit mat.+ clement (TJ= 375 cm-‘) [SJ. AE& and q are- three times smaller and lower than for H&O, respectively_ C&S seems to belong to an intermediate molecule case because the Tt state formed _by direct excitation does not phosphoresce but decays by a non-dissociative int&nolccul& process-~ If so, it would bc reasonable to assume a weak coupling model for intermediate molecuIes_ Suppose that each vibrational level of S, interacts weakly with a single Tf level_ Here the double dagger reminds us that the TX level, lying in the energy region of S,, is vibration&y excited A&&ing to Avouris et & flS]. the moleculti eigenfunction corresponding to such a pair car-be written as ’
-.
.
#c= V/A-E, zind ti=
7s = l/[
(1- p)kS + kZ].
rL = I/(&+
and Z>Zz
kL).
(4)
= p/(1 - p)_
in table 2, the values of rs decrease with increasing photon energy wt, while the values of rr increase_ This fact indicates that the coupling parameter p decreases with oL. The decrease of the coupling parameter may be explained by the change of Franck-Condon factors between the singlet and triplet states with increasing viiraAs
tional
‘.-.*
7-.
..-.
shown
lev&
Ackno&dgement The authors wish to thank T_ Suzuki for his help in the experiment and Drs_ M. Koizumi and A. Tachibana for sending us the results of their IRC analysis prior to publication. This work was partly supported by a Grant-in-Aid of the Ministry of Education, Science and Culture.
_Referen&
.-
m_.~
~~ ~--
._.::_1_:;‘,--
-.I. :
_
‘:
_:.;
.. 111ES
Yeuag and C-B. I&ore,
J. Chk_
Phys. -5s (r97$
398s.
[2] U KawasakJ. K Ka&aui, Y. Ogawa and i . Sate. Chem. Phya 74 (1983) 83.
13)J-R. McDonald z&d L_EBrus.
Chem Phya Let& 16. (1972).687. [4] R-H_ Judge and G-W. King. Can J_ Phy+. 53 (1975) k27; RK Judge and G-W: King. J_ _MoL Spectry_ 74 (1979) 175. [5] M Kawasaki. K Kasatani. S. Tar&m&i. H. Sate and Y_ Fujimura. J. Chem_ Phys. 78 (1983) 7146. 161DJ_ Clouthier and DC Moule, J_ &4oL Speetry_87 (1981). 471.
[7] RP. Steer.Rev_Chem.Intermediates 4 (1981)1; DJ_ CJoufhier. PA Ha&e& AR Knight and R.P_ Sker. J. Photochem_17 (1981) 319. [S] DD. Moule and C-R Sub mmaniau. J. MoL Spauy. 48 (1973) 336. (91 S-II_Lin. Proc Roy. Sot A352 0976) 57. [LO] AE Bruno aad P.R. Steer. J_Chem Phyr 78 (1983) 6660. [II] P_J_Robinson and KA_ Holbrook inr UaimokcuJar reactions (WiJey-Jncerxience, New York. 1972) eq. (S-38). [12] P-H Turner. I- HaJoneu and LM MiBs. J_ MoL Specuy_ 88 (1981) 402_ [13] A Taehiiana. I_ Ohazaki. M Koizumi. K HOI-Jand T_ Yamabe. to be published_ [14] P_ Ho. D_L Barnford. RJ. Buss. Y-T_ Lee and C_B_ Ivloo~ J. Chem_ Phys_ 76 (1982) 3630. [IS] W-H -Xi&r. J_ Am_ C&m_ Sot 101 (1979) 6.910.. 1151BA Waite. SK_ Gray and W-H Miller. J. Chem Phya 78 (1983) 259_ [17] EKC. Lee and G-J- Loper. ix Radiationless traaaitious, ed. SK Liu (Academic Press, New York, 1980) p_ 1. [18] P. Ayourk W_M Gdbert and MA El-Sayed, Chm Rev77 (1977) 793.