- Email: [email protected]
Intersystem crossing in glyoxal vapor. An experimental study of both the resonant and statistical limit
r
CHEMICAL PHYSICS LETiElW
INTERSYSTEM AN
CROSSING
EXPE.RIMENTAL RESONANT
IN STUDY
AND
15 January
CZYOXAL QF
’
VAPOR.
BOTK
STATISTLCAL
2971
THE
LIMIT*
Received 24 November l9?0
We des’cribe results from a continuing study of excited state rel&~aticn processes in @yo~~lr @XiCXXO~ChatdemcmsQat$ the reSonZnt or small mobx& limit of a nonradiat&ve transitfon. fntersystem crossing from the excited singlet (I&) state to a triplet state (presumably the %u state) is shown to be collisionall_vinduced. aed thus glyoxal provides an example beyond the realm of triatomics where the re~alfantlimit predicted by theory is clearly dismayed. On ths other hand, the data show that intersystem crossing from the tripl& tcrthe ~ouad state, il present, is intramolecular. This is
consistent with predictions that this transition lies in the statistical
Emission from both the singlet (IA,) and the triplet (3Au) state of glyoxai has been induced by ‘excitation
of the 1Au state
with 4358
. radiation fro&~ a ‘low pressure Wg aret.
The
On the other hand, the bottom spectrum of fig.1 shows that only singlet emission is visible at very low pressure& Thus, as the frequency of coilisiondl interaction with the excited states increases, the spectrum changes from 0nIy singlet emission to a mixture of singlet and tripfet emission, and finally to triplet emission dlone.~ These transformations are cbnsistent with the predictfans of the theory of nonradiative transitions between the various electronic states of a molecule. Our experiments can be qualitatively described by simply classifying the possible nonradiative transitions of g&&I into either the resonant (or small molecule) limit or the stati&tical (or large moleeuk) limit. in the former, collisions are required to effect a transition, and in the Iatter (the statistical limit} the.~~s~~on is. an ~~arnoIe~~r phenomenon whose rate may be virtually ~ndep~d~t of collisional pertur@tion. The hsis of such classification in glytuial lies in calculation of the density of vibrational levels in the final eIectror&c state of the transition. For the singlet to triplet (IA,, aAU) intersystem ctiossing f&m the region of the 1Au state populated by the absorptjon act, one must know. t&e density @). of tie triplet UibrationaX
A
refatiive intensities of those emissiolbs are sew
sitive to colXlsiod Erequency, and in this commtrnication we shall discuss the relationship of this sensitivity to the nonradiative decay of the excited states. The effect of increasing collision frequency on the intensities of singlet and triplet emission may be seen be examining the spectra in fig-l. As oyefohexane is added to 0.2 torr of glyoxal, the spectrum undergoes a marked transformatian. This is shown in the upper three spe&ra. Singlet emission is quenched by added gas, while there is a slight increase in triplet emission intensity. At added gas ptiessures Eden higher than used for fig,& siliglet emission disappears complete~~y, and only triplet emise&n remains. This behavior is also seen with ^added He or Ar, and therefore it is not specific . to the nature af the oaifision partnert * Coaklbutian No. X883 from.the Chemical Labora‘tories of Indiana Ifnlversity, - ** Present address: S. f, L, Ai, University of Cdoiado. Boulder; Colorada 80302. USA.
that this ekciting: line pu&ps.a +l&attto& level I.. that lies about lio0 &i-l above the zero p&at .ievel of tfre Q&sfste og @pxziL -. -1
limit.
:
.
-
8, number 2
Vohme
CHEMICAL
PHYSICS
LETTERS
15 January 1971
density of states is so high that almost certainly
pV >>1 in each cause. This places these transi-
the statistical Limit, andsuchtransitions in other molecules have been shown to be nearly insensitive to the magnitude of collisional interactions with the initial states [lo, 111. These predictions of a collision induced crossing and a colfision IA, -+3&u intersystem insensitive ~Au 4 lAg intersystem crossing describe our results weltl. At low pressures, tions h
-
-
-
0.2Gt
29.2CH
where
collisienal
is small, sion
interaction
few triplets
v&h the 1.4~ state
can be formed
and emis-
is observed from only the singlet state. At
higher pressures,
c&J&ions
induce
szrfficient
describe
the con-
IA u -+ 3Au crossing to yield obervable emission from the 3Au state. At even higher pressures, the crossing rate becomes so fast that radlatve decay of the singlet state is no longer competitive. Accordingly, singlet emission disappears and triplet emission dominates the spectrum. The predictions of collision induced nonro-diative decay of the IAu state snd collision insentive
_ I
3Au relaxation
stant triplet emission presence of declining
1
T(O.0~
i
19000
1
20000
q,Jhi: *:7:J Lo ,
1
2tooo
I
22ooocM-~
Fig. 1. Emission from gIyoxal vapor excited with the 4358 A Hg line. Triplet emission is to the left near 19290 cm-l. Singlet emission is prominentbetween 19500 and 22700 cm-l. The bottom spectra are from pure glyoxal (CI) while the upper spectra are from glyoxal in the presence of added oyclohexane (CH), The pressures are indicated in t&r. Scattered exciting radiation would lie just off scale to the right, and other scattered mercury lines occur at the positions of the breaks in the spectra.
4@00 cm-l above the triplet zero point level#:. We calculaie this to be not more than about six states per cm-l*. Al‘tude of the matrix elethough the average ma?u ment (V) coupling the Au and the 3Au states is not known, the exceedingly small value of p indicates that the product pV GCL This is the criterion for the resonant knit, and one there: fork expects that the lAu -+ 3AU crossing in glyoxal will be col&ion&lly induced. Ori thk othe+ hand, high densities of final vibratiktl levels exist for the other possible nonr&Bative transitions in glyoxal. For the lAu’* lAg internal conversion, px 2 x 107 . states per em-l, a&d for the 3Au -t 1Au i&&r.system crddng, p=p x 103 states per cm-l. Again the magnitude of v is not known, but the ._ _ -. . : . _ ., :; __. levels
lying
shout
also
intensity observed in the singlet i&-ertsi~ at high
pressures. First, collision induced processes, incUing triplet formation, dominate the relaxation of the IAu state at high pressures. This forces the triplet forndiorr I-ieId to a constant limit at hig’n coiksion frequencies, whereas the yield (and hence the intensity) of singlet emission continues to decline. When this ia coupled with the expectation that triplet nonradiative decay is collision independent, the prediction of constant triplet sures results.
emission
intensity
at high pres-
# 7ye ignore the presence of the lBg and 3Bg states that lie close to their AU counterparts. Although tbey may participate in the excited state relaxation, they have not yet been observed expertmentally. Their lncluslon may mOdifv some detalla of rekktion, but it will not-bear a&nbXcantly on the ofa&fkation of the nonradiative transltione fnto the large or amalI molecule LImita.
*The triplet vihrtional frequencies (same frequencies are given by Goetz fSf, the other were estlmated)were
approximated by two doubly dege-
nerate andeight nondegenerategroups. The ground &ate vibraticnal frequencfes fS] were approxi-
mated by sfx dotoublgdegenerate groups. The densi2y calouIatIon for the ZAu -. IAg transition WE made for levels Withabout 23X% cm-l of vlbratiorial energy and that for the SAu-xAg transition assumeda vibrtiondl energy about 19200 oar1 above zero polot. Not all fInaI vibratiooal states
can effecttvely’cmp~e to f&e initial state of tkse transiticnw [SJ 60 f&se eat$ates are upper
bounds:.
,233 -_-
,
.
‘._
~Obime 8. number.2 .
-
-.
Flash &citation
CHEMXCAL P~rysxs
exper$ments hate been used ~s~sftivity of the tiiplet lifetime to added gases. The lifetime in 0.2 torr of glyoxaf is.0,%6-msec, and it-is unaffected by added &yciohexane or argon, l&fortunately we are *able to state with certainty that intersystem crossing from the triplet to the’ &round state actually occurs: Although other. work ‘at high&= glyoxti pressures suggests that ii does [IZ], careful quantum yield measurements under the conditions of the present exptiriments arq r&q&red to confirti it. We have also obtained from flash excited lifetime measurements a rough estimate of the efficiency of Collisions-in destroying the ‘Au ~ state of glyoxal.. The cross section for collisional destruction by glyoxal-glyoxal collisiotis appears to be very Mgh. ft is within an order of magnitide.of gas kinetic CI’USSsections. The appearance of visible triplet emission at very low,pressw&s suggests that much of,this cross section is associated with intersystem crossing, and possibly this may be the only collisional destruction of ttie’ lAu stat_e. However, there ‘is some photochemical relaxation at high gly, oxal pr&sur& [12]. Whether or not it makes significant contribution to iA, relaxation under the conditions of the present study is a question ’ that can’ be answered only after further work. The possibility of cbemiaal relaxation thus complicates the quantitative interpretation of experimentij with glyoxal such as these. However, conclusions about the respective resonant and statistical behavior of nonradiative relaxation o&the 1~~ and 3Au states’ stands aparte from to make a check of t&i predicted
LETTERS
1; Jmturyl971
the uncertain@s associated with chemical _ relaxation. These he cfearjy described by the ,emission data.
This’
research was ma& pos&le by funds from the Natisnal Science Foundation.
fl] J. Paldus and D.A.Ramsay, Can. J. Phys. 45 (1961) 1389; F. W. Eras, J. M, Brown, A. R, H. Cole, A.Lofthus, S. L. N. G. Krfs~~acb~i, 0. A-Osborne, J.Paldus D.A. Ramsay and L. Watmann, Can.J. Phys. 45 (197 1230. [2J J.Y. p. Brand, Trans. Faraday Sot. 50 (1954) .
”
(41iv. H&w and D. A.Ramsay, Can. J. Phys. 48 11970)1759. [5] G. W. Robinson, 5. Chem. Phys. 47 (1967); G, W,Robineon and R. P. FIX%&,J. Chem, Phys. 38 (2963) 1187; 37 (1962) 1962, [6]3.Jortner and R. S. Berry, 3. Chem. Phys. 48 . (l96S)2757; M. l3ixon and J.Jorkrter. J. Chem. Phvs. _ 4s (i9sa) 7x8. 171J. Jortner, S. A. Rice and R. M. Hochatrasser, AEtPan.Photochem. 7 (1969) 149. [S] W. A. Goetz. A_J. McHugh and D. A.Ramsay, Can. J. Phva. 48 fl970) L. [9JR. K. krris; Spekrachim. Acta 20 (1964) 1129. ltOlC.8. Parmenter and A. H. White. J. Chem. P&s. _
60 (iS§B)
1631.
ill] C. S. Fomenter and ELM. Polzmd, J. Chem. Phys. 51 11969)1551. [I21 C. S. Parmenter, J. Chem. Phys. 41 (l964) 658.
CHEMICAL PHYSICS LETiElW
INTERSYSTEM AN
CROSSING
EXPE.RIMENTAL RESONANT
IN STUDY
AND
15 January
CZYOXAL QF
’
VAPOR.
BOTK
STATISTLCAL
2971
THE
LIMIT*
Received 24 November l9?0
We des’cribe results from a continuing study of excited state rel&~aticn processes in @yo~~lr @XiCXXO~ChatdemcmsQat$ the reSonZnt or small mobx& limit of a nonradiat&ve transitfon. fntersystem crossing from the excited singlet (I&) state to a triplet state (presumably the %u state) is shown to be collisionall_vinduced. aed thus glyoxal provides an example beyond the realm of triatomics where the re~alfantlimit predicted by theory is clearly dismayed. On ths other hand, the data show that intersystem crossing from the tripl& tcrthe ~ouad state, il present, is intramolecular. This is
consistent with predictions that this transition lies in the statistical
Emission from both the singlet (IA,) and the triplet (3Au) state of glyoxai has been induced by ‘excitation
of the 1Au state
with 4358
. radiation fro&~ a ‘low pressure Wg aret.
The
On the other hand, the bottom spectrum of fig.1 shows that only singlet emission is visible at very low pressure& Thus, as the frequency of coilisiondl interaction with the excited states increases, the spectrum changes from 0nIy singlet emission to a mixture of singlet and tripfet emission, and finally to triplet emission dlone.~ These transformations are cbnsistent with the predictfans of the theory of nonradiative transitions between the various electronic states of a molecule. Our experiments can be qualitatively described by simply classifying the possible nonradiative transitions of g&&I into either the resonant (or small molecule) limit or the stati&tical (or large moleeuk) limit. in the former, collisions are required to effect a transition, and in the Iatter (the statistical limit} the.~~s~~on is. an ~~arnoIe~~r phenomenon whose rate may be virtually ~ndep~d~t of collisional pertur@tion. The hsis of such classification in glytuial lies in calculation of the density of vibrational levels in the final eIectror&c state of the transition. For the singlet to triplet (IA,, aAU) intersystem ctiossing f&m the region of the 1Au state populated by the absorptjon act, one must know. t&e density @). of tie triplet UibrationaX
A
refatiive intensities of those emissiolbs are sew
sitive to colXlsiod Erequency, and in this commtrnication we shall discuss the relationship of this sensitivity to the nonradiative decay of the excited states. The effect of increasing collision frequency on the intensities of singlet and triplet emission may be seen be examining the spectra in fig-l. As oyefohexane is added to 0.2 torr of glyoxal, the spectrum undergoes a marked transformatian. This is shown in the upper three spe&ra. Singlet emission is quenched by added gas, while there is a slight increase in triplet emission intensity. At added gas ptiessures Eden higher than used for fig,& siliglet emission disappears complete~~y, and only triplet emise&n remains. This behavior is also seen with ^added He or Ar, and therefore it is not specific . to the nature af the oaifision partnert * Coaklbutian No. X883 from.the Chemical Labora‘tories of Indiana Ifnlversity, - ** Present address: S. f, L, Ai, University of Cdoiado. Boulder; Colorada 80302. USA.
that this ekciting: line pu&ps.a +l&attto& level I.. that lies about lio0 &i-l above the zero p&at .ievel of tfre Q&sfste og @pxziL -. -1
limit.
:
.
-
8, number 2
Vohme
CHEMICAL
PHYSICS
LETTERS
15 January 1971
density of states is so high that almost certainly
pV >>1 in each cause. This places these transi-
the statistical Limit, andsuchtransitions in other molecules have been shown to be nearly insensitive to the magnitude of collisional interactions with the initial states [lo, 111. These predictions of a collision induced crossing and a colfision IA, -+3&u intersystem insensitive ~Au 4 lAg intersystem crossing describe our results weltl. At low pressures, tions h
-
-
-
0.2Gt
29.2CH
where
collisienal
is small, sion
interaction
few triplets
v&h the 1.4~ state
can be formed
and emis-
is observed from only the singlet state. At
higher pressures,
c&J&ions
induce
szrfficient
describe
the con-
IA u -+ 3Au crossing to yield obervable emission from the 3Au state. At even higher pressures, the crossing rate becomes so fast that radlatve decay of the singlet state is no longer competitive. Accordingly, singlet emission disappears and triplet emission dominates the spectrum. The predictions of collision induced nonro-diative decay of the IAu state snd collision insentive
_ I
3Au relaxation
stant triplet emission presence of declining
1
T(O.0~
i
19000
1
20000
q,Jhi: *:7:J Lo ,
1
2tooo
I
22ooocM-~
Fig. 1. Emission from gIyoxal vapor excited with the 4358 A Hg line. Triplet emission is to the left near 19290 cm-l. Singlet emission is prominentbetween 19500 and 22700 cm-l. The bottom spectra are from pure glyoxal (CI) while the upper spectra are from glyoxal in the presence of added oyclohexane (CH), The pressures are indicated in t&r. Scattered exciting radiation would lie just off scale to the right, and other scattered mercury lines occur at the positions of the breaks in the spectra.
4@00 cm-l above the triplet zero point level#:. We calculaie this to be not more than about six states per cm-l*. Al‘tude of the matrix elethough the average ma?u ment (V) coupling the Au and the 3Au states is not known, the exceedingly small value of p indicates that the product pV GCL This is the criterion for the resonant knit, and one there: fork expects that the lAu -+ 3AU crossing in glyoxal will be col&ion&lly induced. Ori thk othe+ hand, high densities of final vibratiktl levels exist for the other possible nonr&Bative transitions in glyoxal. For the lAu’* lAg internal conversion, px 2 x 107 . states per em-l, a&d for the 3Au -t 1Au i&&r.system crddng, p=p x 103 states per cm-l. Again the magnitude of v is not known, but the ._ _ -. . : . _ ., :; __. levels
lying
shout
also
intensity observed in the singlet i&-ertsi~ at high
pressures. First, collision induced processes, incUing triplet formation, dominate the relaxation of the IAu state at high pressures. This forces the triplet forndiorr I-ieId to a constant limit at hig’n coiksion frequencies, whereas the yield (and hence the intensity) of singlet emission continues to decline. When this ia coupled with the expectation that triplet nonradiative decay is collision independent, the prediction of constant triplet sures results.
emission
intensity
at high pres-
# 7ye ignore the presence of the lBg and 3Bg states that lie close to their AU counterparts. Although tbey may participate in the excited state relaxation, they have not yet been observed expertmentally. Their lncluslon may mOdifv some detalla of rekktion, but it will not-bear a&nbXcantly on the ofa&fkation of the nonradiative transltione fnto the large or amalI molecule LImita.
*The triplet vihrtional frequencies (same frequencies are given by Goetz fSf, the other were estlmated)were
approximated by two doubly dege-
nerate andeight nondegenerategroups. The ground &ate vibraticnal frequencfes fS] were approxi-
mated by sfx dotoublgdegenerate groups. The densi2y calouIatIon for the ZAu -. IAg transition WE made for levels Withabout 23X% cm-l of vlbratiorial energy and that for the SAu-xAg transition assumeda vibrtiondl energy about 19200 oar1 above zero polot. Not all fInaI vibratiooal states
can effecttvely’cmp~e to f&e initial state of tkse transiticnw [SJ 60 f&se eat$ates are upper
bounds:.
,233 -_-
,
.
‘._
~Obime 8. number.2 .
-
-.
Flash &citation
CHEMXCAL P~rysxs
exper$ments hate been used ~s~sftivity of the tiiplet lifetime to added gases. The lifetime in 0.2 torr of glyoxaf is.0,%6-msec, and it-is unaffected by added &yciohexane or argon, l&fortunately we are *able to state with certainty that intersystem crossing from the triplet to the’ &round state actually occurs: Although other. work ‘at high&= glyoxti pressures suggests that ii does [IZ], careful quantum yield measurements under the conditions of the present exptiriments arq r&q&red to confirti it. We have also obtained from flash excited lifetime measurements a rough estimate of the efficiency of Collisions-in destroying the ‘Au ~ state of glyoxal.. The cross section for collisional destruction by glyoxal-glyoxal collisiotis appears to be very Mgh. ft is within an order of magnitide.of gas kinetic CI’USSsections. The appearance of visible triplet emission at very low,pressw&s suggests that much of,this cross section is associated with intersystem crossing, and possibly this may be the only collisional destruction of ttie’ lAu stat_e. However, there ‘is some photochemical relaxation at high gly, oxal pr&sur& [12]. Whether or not it makes significant contribution to iA, relaxation under the conditions of the present study is a question ’ that can’ be answered only after further work. The possibility of cbemiaal relaxation thus complicates the quantitative interpretation of experimentij with glyoxal such as these. However, conclusions about the respective resonant and statistical behavior of nonradiative relaxation o&the 1~~ and 3Au states’ stands aparte from to make a check of t&i predicted
LETTERS
1; Jmturyl971
the uncertain@s associated with chemical _ relaxation. These he cfearjy described by the ,emission data.
This’
research was ma& pos&le by funds from the Natisnal Science Foundation.
fl] J. Paldus and D.A.Ramsay, Can. J. Phys. 45 (1961) 1389; F. W. Eras, J. M, Brown, A. R, H. Cole, A.Lofthus, S. L. N. G. Krfs~~acb~i, 0. A-Osborne, J.Paldus D.A. Ramsay and L. Watmann, Can.J. Phys. 45 (197 1230. [2J J.Y. p. Brand, Trans. Faraday Sot. 50 (1954) .
”
(41iv. H&w and D. A.Ramsay, Can. J. Phys. 48 11970)1759. [5] G. W. Robinson, 5. Chem. Phys. 47 (1967); G, W,Robineon and R. P. FIX%&,J. Chem, Phys. 38 (2963) 1187; 37 (1962) 1962, [6]3.Jortner and R. S. Berry, 3. Chem. Phys. 48 . (l96S)2757; M. l3ixon and J.Jorkrter. J. Chem. Phvs. _ 4s (i9sa) 7x8. 171J. Jortner, S. A. Rice and R. M. Hochatrasser, AEtPan.Photochem. 7 (1969) 149. [S] W. A. Goetz. A_J. McHugh and D. A.Ramsay, Can. J. Phva. 48 fl970) L. [9JR. K. krris; Spekrachim. Acta 20 (1964) 1129. ltOlC.8. Parmenter and A. H. White. J. Chem. P&s. _
60 (iS§B)
1631.
ill] C. S. Fomenter and ELM. Polzmd, J. Chem. Phys. 51 11969)1551. [I21 C. S. Parmenter, J. Chem. Phys. 41 (l964) 658.