- Email: [email protected]
The luminescence excitation spectrum of γ-irradiated ethanol glasses at 77 K. Presence of an anion band; H−
Volume 35, number
3
CHEMICAL PHYSICS LETTERS
THE LUKWESCENCE
EXCWATION
1.5 September
SPECTRUM OF y-IRRADIATED
1975
ETHANOL CLASSES AT 77 K.
PRESENCE OF IAN ANION BAND: RT.B. TRUONG Eqkpe de Reclwrcke C$RS no. 98, Laboratoire Centre d’Orsav, 914’05_Orra,v, France Received 11 hkxrch 19.75 Revised mxnurript received
dc Chimie Phvsique, Universic~ P&s Sud,
5 June 1975
7-k = 1000 nm band observed in the luminescence excitation spectrumlsL =j’(hb), of -y-irradiated EtOH &ss, H20(&,0) polycry:kAine ices, alkalines ices at 77 K 3s well as in the absorption spectrum of irradiated EtOH glass at 4 6 might be ascribed to H-.
over dinitrophenylhydrazine
1. Introdwtion The luminescence of y-irradiated alcohols at low temperature, up to the present time, has been observed only-by thermoluminescence [1,2J while that of the irradia!ed alkanes has b?ec studied intensively by means of thermoluminescence, isothermoluminescence or stimulated lumescence (SL) Cl,3 1. The latter method is a sensitive one. The enrission spectrum, 1s~ = f&), obtained cpon bleaching the trapped electrons in the irradiated sample at a constant wavelength hb serves to identify the neutralized cation emitter [S-S], while the luminescence excitation spectrum, IsL =f&), @es information on the negative species (e; or anion) stabilized in the matrix [ 1,3]. These negative species might not be detected by optical absorption measurements because of their low concentration or by EPR because of a non-paramagnetic character. We now present the luminescence eXCitatiOn
SpeCtrUm,
IS,
=f@b)
of -y-irradiated
ethanol (EtOfi) glasses at 77 K. To our knowledge, this is the first stimulated luminescence spectrum ever reported for y-irradiated EtOH. The presence of an SL anion band wkch mighht be ascribed to H- is also
and redistilled over a mixture of magnesium-iodine to remove traces of water. The purified EtOH has 100% transmission at 200 nm and gives no luminescence upon excitation with X = 2.54 nm under maximum sensitivity of our luntinescerkze detection system. !&tails 3f the’technique of stimulated luminescence (SL) have heen described elsewhere [3 1. &gassed purified EtOH was sealed in suprasil tubes (i.d. = 3 mm) and irradiated at 77 K with a %o source ar dose 1018 - 101’ eV g-l. The SI, was observed simultaneously during excitation of the irradiated samples
with X, varying
from 2000
to 400 nm
and always starting at the longer wavelength. By this arrangement, even short-Lived emission with low intensity could be detected. It has been reported that the emitting centers in Firradiated EtOH glasses are not associated with impurities or radiolysis products; the transition from the
lowest triplet to the ground state of the molecule EtOH itself upon charge neutralization is responsible for the observed emission at 34.5 nm in irra&ated EtOH gasses at 77 K [5].
!&cussed.
3. Results 2. Expetimentd EtOH, a Merk product,
Curve i in fig. 1 displayes
was purified by distillation
the I,,
= Jc(hb) of r-ir-
radiated Et%! glasses at 77 K by monitoring X, =
Volume
CHEMICAL PHYSICS LETTERS
35, nurnbcr 3
313 nm, W dose: 10” - 10”
1s~ (arbitrary unit)
I
time
Fig. 1. The hxninescenw t%CitJtiOn Spcctrunl, 1s~ = I(&). 77 K. Curve 1 - Et011 ~lasscs, A,, = 345 nm [5]; rdose: lOI eV g-l ; curve 7 - polycrystaltine Ii20 , Aan = 380 nm [4], ydose: 2 x DJ~ ev g-1 ; CUrYe3 - polycrystallinc 2 X 10” eVg_‘:curve 4 DZ 0, h,n = 380 nm [4],-+xe: photoionized tryprophan in Hz0 ice; curvr, 5 - photoionized
at
tryprophan in D20 ice. (k,, = 3 I3 e 10 nm From an OSRAM HBO 500 W, UV dose: LOL8 hv an2 s-l, irrndiation
time: 10 s, post irradiation
time: 20 s, h,,
= 440 nm, A.,,,
of tryptoplian phospho~:escence band.) Curve 6 - alkaline ice (10 hI NaOH) 7dow: 5 X 10lg eV g-’ kan = 380 nm [I
I].
345 nm, the X,, oi‘the stimulated emission curve [5]. The visible region part of curve 1 lviith a threshold at = 550 nm, is similar to the reported spectrum giving relative quantum yield 4,~ =f(Xb) for the photobleaching
of e:; in y-irradiated
!S September 1975
EtOH
glass
at
low temperature [6,7]. The fsL=J&,) of EtOH at 77 K shows some re:;emblance to the ones of y.irradiated Hz0 (curve 2, fig. 1) and D,O (curve 3, fig. 1) ices [3] with a sligh;: b!ue-shift for the alcohol. Compared to the irradiated ices, the stimulated luminescence in irradiated E3tOH glass is more intense and its intensity is more reproducible; the I,, band in the IR region (* 1000 n.m) is much more pronounced. A band in the same spectral region was observed by Namika et al. [g] al 4 K by optical absorption measurements. These au,rhors have attributed this z 1000 nm band along with the 15c10 nm band to shallow, i.e., unrel&Ked traps in irradiated EtOH glasses. Attempts to obtain the 1~~ = i&) of photoionized N,N’,N,N’-tetmmethyl-p-phenylenediamine (TMPD), tryptophan or tyrosine (10w3-- 1O-’ M) in EtOH glasses at 77 K were unsuccessful (Xe,, = 254 or
l-30
s); even
though,
hu cm2 s-l
intense
; irradiation
isotherm21
lumin-
escence lasting several minutes after the UV cxcitation is observed. The result is consistent with the observation of Eernas and Grand [9] who reported that no trapped electrons (et) were observed in photoionization of solid solutions of TMPD or DPPD (diphenylp-phenylencdiamine) in EtOH glasses a~ 77 K. The e; were probably formed and simultaneousi:, photodetached from their traps by the UV excitation wavelength. Since at Xesc = 254 or 313 nm the extinction coefficient of et in EtOH at 77 K is still high: 5 X lo3 [9]. We also observe an IR band in the same SL spectral region folIowing7-radiolysis of H,O, D20 polycrystalline ices [3] (curves 2 and 3, fig. 1) or of aqueous alkaline glasses [lo (curve 6, fig. I) at 77 K (y dose: 10” - 10” eV g- 1). This =Z1000 nm band does not exiist in the fsL = f&) of photoionized ttyptOphn (10B3 M) in the same matrices at 77 K. The stimulated luminescence inrensity of photoionized tryptophan in H,O, D20 ices at 77 K (curves 4 and 5, fig. 1) is very weak as compared IO the one in aqueous alka!ine glasses [IO]. The ~1000 nm SL band in r-radiolysis of H,O, D20 polycrystalline ices [3], of aqueous alkaline glasses [lo] or of EtOH glasses at 77 K is stable for at least two or tluee hours in the dark.
4. Discussion 4, I. The ;=1000 lz?n SL band is not associgterl s~1~11low traps
with
The presence of the = 1000 nm band in the ISL = f&) of y-irradiated puIe so!vents (curves I,2,3 and 6, fig. 1) and its absence in the IsL = f&,) of photoionized tryptophan in the same matdces at 77 K appear to imply that the = 1000 nm SL band is not associated with shallow traps. As for the irradiated EtOH glasses, the observed isotherma! luminescence after W excitation indicates that photoionization had effectively taken place. The absence of the = 1000 run SL band in the photoionizztion cases implies either that: with the (a) The et localized in cavities associated 1000 nm band are bleached off, like the et related to 427
Volume 35, number 3
the 540 nm band, by the W excitation wavelength. This would be uniik-k!r, since krkey et al. [7] have reported that when EtOH glasses were y-irradiated at 77 K and were bleached with Xb d 436 nm at 6 K, an IR band appeared in the absorption spectrum. The reported result indicates that the UV excitation wave= 254 or 313 nm) would not remove the with the Z=IGOO nm band. (b) Or the = 1000 nm band is not related to shallow traps. Teply il l] has reported that the et traps in r-irradiated EtOH glasses at 77 K are practically fully relaxed. length &.,
e; associated
The-absence of,the IR stimulated luminescence .band in photoionization cases and its presence in yradio!ysis cases, therefore, indicate that the Z=1000 nm SL band might be either a dielectron band or an anion band, a radiolyzed pro duct of irradiated EtOH glasses, H20@,0) polycrystalline ices and of irradiated alkaline ices. The former would be improbable because of the low -i-doses used. Besides, the dielectron cen$ers have not been observed in irradiated neutral aqueous solids and alcohol glasses and are probably not stable in these matrices [ 121. The anion band then appears more plausible. The experiment of Namiki ct al. [8] in irradiated EtOH gIasses at 4.K is consistent with our suggestion. By optical absorption rr%a:;urements t!!cse authors observed that with hb = 1000 nm, at 4 K, the 1000 nm band was removed mor: rapidly than the 1 SO0 nm band. The result indicam that two types of traps: physical and chemical, .are present. In fact, if both bands (IO00 and 1500 11rn) were related to shallowly trapped electrons in sinplc-component glzsses, a bleaching photon of 1.:!4 eV (1000 nm) wouldremove the e!ectrons from the mo?e shallow traps (1500 run band) faster than the ones from the less shallow traps (1000 run). The dent observed at the 1000 nm region upon hb = 1000 nm in ::ef. [8] may be interpreted by a transfer of the photodetached electron (e;) from the ICOC) nm traps to the 1500 nm traps. Due to strong overtappiug of these balds, this transfer would, however, not be observable until the 1000 nm hsd been
sufficiently bleached off (curve c ti fig. 2 in ref. [S]). By using X, 5 436 nm i:in the et band) Perkey [7] observed that the I13 band was regenerated. 428
IS September 1975
CHEMICAL PHYSICS LETTERS
et 4.
,&ashimura 1131 also reported that a slight increase of the visible band was observed when bleaching in the IR region. This type of reversible electron transfer or eIectron shutt!ing is typical between chemical traps (10CO nm band) and physical traps (540 nm band) in single-component matrices. It has been suggested previously that the SL band in the IR region observed in y-irradiated HZ0 (DZO) ices might be an anion band [ 141. 4.3. A H-
bmd?
A common anion formed upon radiolysis of EtOH, H,O(D,O) or of aqueous alkaline solution would be either OH- (OD)- or H- (D-). Since the photoionzed solid solutions of tryptophan or of tyrosine in 5 to 10 M of [OH-] do not show any SL band in the IR region [IO], H- would be likely the one. The = 1000 nm SL band might then be correlated with H- (D-). In addit‘ Ion, it is interesting to note that the photodetachment threshold of the z 1000 nm SL band is x0.8 eV, a value which is more appropriate to the electron affi.tity (EA) of the H atom (0.7 eV in the pds phase) than of the OH radical (1.8 eV). In the solid phase one might expect the EA to have a larger value [l]. 4.4. Essq to interpret the experimental results irz y-irradiated pwe solvent glasses with the presence of an =I000 run H- band
The fact that H atoms have not been recorded in y-irradiated EtOH glases even at very low temperafure (4 K) has been puzzling [lS]. The presence of H- in y-irradiated EtOH glasses might accour~t for this abnormality. From the rewlts of ref. [7], after the IR absorption band in r-irradiated EtOH glasses at 4 K was bleached off, with Xb = 500 nm no IR band reappeared, and from the resulti of ref. [8] : the same observations were made at 6 K; with hb G 436 run, however, the IR absorption band was reproduced, one might guess that the blue tail of this IR band (H-) would extend into the visible region and probably ends around 44d nm in solid &OH.
When using Xb > 440 nm, one would photodetach the electrons from the 540 nm (et) traps and simultaneousli from the 1009 nm (H-1 traps:
Volume 35, number 3
\ - ~~zcanm -et
e- ./ m
CHEMICAL PHYSICS LE-I-iERS
‘+C,H,OH;-
/’
CzH50H‘+H ,’
(I)-
/’ C2~,0i+
phosphorescence O: EtOH
at
34_5nm
[Sj
Reaction (I) 116,171 competes with the non-luminescent dissociative attachment [18] : (2) and a
small part of t.he e, may a!so be retrapped. The H atoms, thermal or hot, produced in (1) and (2) react lvith EtOH as well-known [16,17] : H+C,H,OH-+CH:jCHOH+H,.
(3)
At low temperature
(4 K, 6 K) where reaction (3) is slow, part of the H :ltoms newly produced during Ihe bleaching may diffuse and act 2s chemical traps for the oncoming
e,
em :
+ H + H- (1000 nm band) .
(4)
This might expiain Ihe results observed at 4 K by Perkey et al. [7] when using X, G 436 nm (i.e., in the e, band) and might account for the absence of H atoms in irradiated EtOH glasses after optical bleaching even at 4 K [ 151. At 77 K where reactions (2) and (3) overshadow reaction (4), the concentration of H- associated with the =lOOO nm band would probably be too low to be detected by optical absorption. H- mi&t possibly be the “ghost X”, Ha precursor, postulated by Russell and Freeman [ 191, which reacts Gth acid to give H2 but not with NzO to give N2 in the radiolysis of liquid EtOH.
H-(D-) has been observed in electron impact on water vapor experiments [20,21 J. Low e; concentration and higher proton mobility in ices might explain the low intensity of the iR band in the 1~. =f&) spectrum of y-irra 3iated Hz0 or D,O ices [3]. 44.3. MTHF glass es The presence of a H- bnnd in the IR optical absorption spectral region in r-irradiated cY-methyltetrahydrafuran @lTH F) glasses might explain: (i) The differeilce in optical bleaching behatiour of et in r-irradiated MTHF glasses at 77 K observed by
Walker and May 1221 at 1152,694,633
and 442 run
15 September
1975
when using laser pulse at these wavelengths. The authors reported that with the 11.52 nm laser pulse, there was immediate bleaching at 1152 nm then some sLi$C recovery in absorbance over the first few hundred nanoseconds, then slow delayed bieaching. The same results were observed at 694 nrn with the 694 nm laser pulse. At 633 and 442 nm during the pulse, however, there was no bleaching at all at these \vavelengths, then sIow delayed bleaching occurred with a time constant of 0.1 s. In y-irradiated MTHF glasses at 77 K, trapped
electrons have an absorption maximum at = 1200 nm. The observed immediate bleaching at 1152 nm and 694 nm during the laser pulse, might be interpreted as due to the bleaching of et whose absorption band in MTHF glasses overlaps the H- absorption band. The slight recovery of absorbance at these wavelengths over the first few hundred nanoseconds might be
caused by partial iransfer of mobile electrons to the H- band [probably by reaction (4)] and ihe delayed bleaching afterwards might associate with the bleaching of H- . At 633 nm and 442 run, the e, absorption in MTHF glasses at 77 K is low. The only delayed bleaching reported after these wavelength laser pulses 13-21appears to indicate that H- was probably predominantly affected. (ii) The two photon transition process at z9.50 nm region reported by Kevan [23] in y-irradiated MTHF glasses but not in photoionized TMPG-MTHF glasses at low temperature. One might think that the first photon detaches the electron from H- and the second photon excites the mobile electron directly into the conduction band. Hence, a small temperature depecdence observed for this two photon transition [24] might be explained: H- becomes more mobile at 77 K than at 4 EC_ In photoionized TMPD-MTHF, the two photon transition was not observed because there was no H-. The unXor,m decrease of the e, absorption band in photoionized TMPD-MTHF glasses at 25 K upon blcackng with 1064 nm laser light reported by Hager and Wlard [24] was consistent with this suggestion.
H- was originally suggested then later eliminated by WiJJard [25] to account for the decreaSO in [et] in
hydrocxbon
gasses irradiated at 77 K with large Y429
.
CHEMICAL PHYSiCS LETTERS
Volume 35, number 3
in doses e 10zo eV g-l)_ For our own experiments, irradiated 3-methylpentane or methylcyclohexane glasses at 77 K, we observed that the Sue,) band Z= 1600 nm; extending to e-600 nm) can be txrn2x easily bleached off with A,, = 1600 nm at low y-doses eV g-l), iit large r-doses p 102’ eV (lO’QO’9 g-l), with Xb = 1600 nm, however, a weak SL band all:fays remains peaking at 1100 nm (intensity ratio to that of the Suet) band: l/20). This might probably be the (H-j SL band in r-irradiated hydrocarbon
glasses at 77 K. Due to strong overlapping of the dominating et band, the H- band in r-irradiated
alkane glasses could not te observed, until [e;] is sufficiently low, that is at y-doses 2 10” eV g-l 1221. Besides chemical xraps such as solvent radicals [3,25], positive ions produced upon y-irradiation [Xl, H atoms could be other additional scavengers
for electrons during the radiolysis of alkanes.
15 September
1975
References
[l] F. Kiefier and 31. hlagat, Actions &I-I. Biol. Radiations 14 (1970). [2] J. Kroh, W. Roszak and Z. Czerwik, Bull. Acad. Sci. 18 (1970) 699. [3] A. Bernas, J. Blabs, AI. Gauthier, D. Grand and T.B. Truorg, Intern. J. Radiative Phys. Chem. 6 (1974) 401. [4] A. Bernas and T.B. Truong, Chem. Phys. Letters 29 (1974) 585. [5] T.B. Truong and A. Bernas, to be published. [6] A. Bernas, D. Grand and C. Chachaty, J. Chem. Sot. Chem. Commun. (1970) 1667. [ 71 LX. Perkey, Parhataziz and R.R. Hentz, Chem. Phys. Letters 27 (1974) 531. [ 8) A. Namiki, hl. Noda and T. Higashimura, Chem. Phys. Letters 23 (1973) 402. [9] A. Bernas and D. Grand, J. Chim, Phys. 67 (1970) 566. [lo] A. Bernas and T.B. Truong, to be pub&he& [l!] J. Teply, Intern. J. Radiative Phys. Chem. 6 (1974) 379. [ 121 B.G. Ershov and -4.K. Piknev, Advnn. Chem. Ser. 81 (1968)
19.
1131 T. Iiigashimura,
H- formed either by: Q+e;-fI-I;(
during
optical
bleaching
or r-irradiation),
(5)
or
Hi c e- + H, (during y.irradiation),
(6)
might constitute another way for the disparition of Hatoms besides: HtH+H2
(7)
and RH+H+R.+H,.
W
Ann. Rcpt. Rcs. Reactor inst. Kyoto Univ. 6 (1973) 38. [I41 A. Bcrnzs and T.B. Truong, Compt. Rend. Acad. Sci. (Paris) 277B (1973) 391. [I51 L. Kevan, Actions Chim. BioL Radiations i 3 (1969). 1161 C. Chachaty and E. Hayon, J. Chim. Phys. 61 (1964) 111.5. [17! F.S. Dainton, G.A. Salmon and J. Tepiy, Proc. Roy. Sot. A286 (1965) 27; F.S. Ddnton, J.P. Keenc, T.J. Kemp, G.A. Salmon and J. Teply, Proc. Chem. Sot. (1964) 26.5. [la1 M.R. Ronayne, J.P. Guarino and W.H. Hamill, J. Am. Chem. Sot. 84 (1962) 4230. [I91 J.C. RusAl and G.R. Freeman, J. Phys, Chcm. 72 (1968) 816. [201 E.N. J&settre and W.M. XUO, J. Chem. Phys. 61 (1974) 1703. [21 I S. Trajmu and R.HalI, J. Phys. B7 (1964) L458. WI D.C. Walker and R. May, Intern. J. Radiative Phys. Chem. 6 (i974) 345. J. Radktikz Phys. Chem. 6 (1974) [231 5. I&van, Intern.
297.
Acknowledgement The author would lik: to thank Dr. A. Bernas for a critical reading of the manuscript. The help in preparing the samples from Mr. A. Petit is also appreciated.
[241 S.L. Hager and J.E.‘iWlard, I. Chem. Phys. 61 (1974) 3244. [251 J.E. Willard, Ictern. J. Radiative Phys. Chem. 6 (1974) 32.5.
3
CHEMICAL PHYSICS LETTERS
THE LUKWESCENCE
EXCWATION
1.5 September
SPECTRUM OF y-IRRADIATED
1975
ETHANOL CLASSES AT 77 K.
PRESENCE OF IAN ANION BAND: RT.B. TRUONG Eqkpe de Reclwrcke C$RS no. 98, Laboratoire Centre d’Orsav, 914’05_Orra,v, France Received 11 hkxrch 19.75 Revised mxnurript received
dc Chimie Phvsique, Universic~ P&s Sud,
5 June 1975
7-k = 1000 nm band observed in the luminescence excitation spectrumlsL =j’(hb), of -y-irradiated EtOH &ss, H20(&,0) polycry:kAine ices, alkalines ices at 77 K 3s well as in the absorption spectrum of irradiated EtOH glass at 4 6 might be ascribed to H-.
over dinitrophenylhydrazine
1. Introdwtion The luminescence of y-irradiated alcohols at low temperature, up to the present time, has been observed only-by thermoluminescence [1,2J while that of the irradia!ed alkanes has b?ec studied intensively by means of thermoluminescence, isothermoluminescence or stimulated lumescence (SL) Cl,3 1. The latter method is a sensitive one. The enrission spectrum, 1s~ = f&), obtained cpon bleaching the trapped electrons in the irradiated sample at a constant wavelength hb serves to identify the neutralized cation emitter [S-S], while the luminescence excitation spectrum, IsL =f&), @es information on the negative species (e; or anion) stabilized in the matrix [ 1,3]. These negative species might not be detected by optical absorption measurements because of their low concentration or by EPR because of a non-paramagnetic character. We now present the luminescence eXCitatiOn
SpeCtrUm,
IS,
=f@b)
of -y-irradiated
ethanol (EtOfi) glasses at 77 K. To our knowledge, this is the first stimulated luminescence spectrum ever reported for y-irradiated EtOH. The presence of an SL anion band wkch mighht be ascribed to H- is also
and redistilled over a mixture of magnesium-iodine to remove traces of water. The purified EtOH has 100% transmission at 200 nm and gives no luminescence upon excitation with X = 2.54 nm under maximum sensitivity of our luntinescerkze detection system. !&tails 3f the’technique of stimulated luminescence (SL) have heen described elsewhere [3 1. &gassed purified EtOH was sealed in suprasil tubes (i.d. = 3 mm) and irradiated at 77 K with a %o source ar dose 1018 - 101’ eV g-l. The SI, was observed simultaneously during excitation of the irradiated samples
with X, varying
from 2000
to 400 nm
and always starting at the longer wavelength. By this arrangement, even short-Lived emission with low intensity could be detected. It has been reported that the emitting centers in Firradiated EtOH glasses are not associated with impurities or radiolysis products; the transition from the
lowest triplet to the ground state of the molecule EtOH itself upon charge neutralization is responsible for the observed emission at 34.5 nm in irra&ated EtOH gasses at 77 K [5].
!&cussed.
3. Results 2. Expetimentd EtOH, a Merk product,
Curve i in fig. 1 displayes
was purified by distillation
the I,,
= Jc(hb) of r-ir-
radiated Et%! glasses at 77 K by monitoring X, =
Volume
CHEMICAL PHYSICS LETTERS
35, nurnbcr 3
313 nm, W dose: 10” - 10”
1s~ (arbitrary unit)
I
time
Fig. 1. The hxninescenw t%CitJtiOn Spcctrunl, 1s~ = I(&). 77 K. Curve 1 - Et011 ~lasscs, A,, = 345 nm [5]; rdose: lOI eV g-l ; curve 7 - polycrystaltine Ii20 , Aan = 380 nm [4], ydose: 2 x DJ~ ev g-1 ; CUrYe3 - polycrystallinc 2 X 10” eVg_‘:curve 4 DZ 0, h,n = 380 nm [4],-+xe: photoionized tryprophan in Hz0 ice; curvr, 5 - photoionized
at
tryprophan in D20 ice. (k,, = 3 I3 e 10 nm From an OSRAM HBO 500 W, UV dose: LOL8 hv an2 s-l, irrndiation
time: 10 s, post irradiation
time: 20 s, h,,
= 440 nm, A.,,,
of tryptoplian phospho~:escence band.) Curve 6 - alkaline ice (10 hI NaOH) 7dow: 5 X 10lg eV g-’ kan = 380 nm [I
I].
345 nm, the X,, oi‘the stimulated emission curve [5]. The visible region part of curve 1 lviith a threshold at = 550 nm, is similar to the reported spectrum giving relative quantum yield 4,~ =f(Xb) for the photobleaching
of e:; in y-irradiated
!S September 1975
EtOH
glass
at
low temperature [6,7]. The fsL=J&,) of EtOH at 77 K shows some re:;emblance to the ones of y.irradiated Hz0 (curve 2, fig. 1) and D,O (curve 3, fig. 1) ices [3] with a sligh;: b!ue-shift for the alcohol. Compared to the irradiated ices, the stimulated luminescence in irradiated E3tOH glass is more intense and its intensity is more reproducible; the I,, band in the IR region (* 1000 n.m) is much more pronounced. A band in the same spectral region was observed by Namika et al. [g] al 4 K by optical absorption measurements. These au,rhors have attributed this z 1000 nm band along with the 15c10 nm band to shallow, i.e., unrel&Ked traps in irradiated EtOH glasses. Attempts to obtain the 1~~ = i&) of photoionized N,N’,N,N’-tetmmethyl-p-phenylenediamine (TMPD), tryptophan or tyrosine (10w3-- 1O-’ M) in EtOH glasses at 77 K were unsuccessful (Xe,, = 254 or
l-30
s); even
though,
hu cm2 s-l
intense
; irradiation
isotherm21
lumin-
escence lasting several minutes after the UV cxcitation is observed. The result is consistent with the observation of Eernas and Grand [9] who reported that no trapped electrons (et) were observed in photoionization of solid solutions of TMPD or DPPD (diphenylp-phenylencdiamine) in EtOH glasses a~ 77 K. The e; were probably formed and simultaneousi:, photodetached from their traps by the UV excitation wavelength. Since at Xesc = 254 or 313 nm the extinction coefficient of et in EtOH at 77 K is still high: 5 X lo3 [9]. We also observe an IR band in the same SL spectral region folIowing7-radiolysis of H,O, D20 polycrystalline ices [3] (curves 2 and 3, fig. 1) or of aqueous alkaline glasses [lo (curve 6, fig. I) at 77 K (y dose: 10” - 10” eV g- 1). This =Z1000 nm band does not exiist in the fsL = f&) of photoionized ttyptOphn (10B3 M) in the same matrices at 77 K. The stimulated luminescence inrensity of photoionized tryptophan in H,O, D20 ices at 77 K (curves 4 and 5, fig. 1) is very weak as compared IO the one in aqueous alka!ine glasses [IO]. The ~1000 nm SL band in r-radiolysis of H,O, D20 polycrystalline ices [3], of aqueous alkaline glasses [lo] or of EtOH glasses at 77 K is stable for at least two or tluee hours in the dark.
4. Discussion 4, I. The ;=1000 lz?n SL band is not associgterl s~1~11low traps
with
The presence of the = 1000 nm band in the ISL = f&) of y-irradiated puIe so!vents (curves I,2,3 and 6, fig. 1) and its absence in the IsL = f&,) of photoionized tryptophan in the same matdces at 77 K appear to imply that the = 1000 nm SL band is not associated with shallow traps. As for the irradiated EtOH glasses, the observed isotherma! luminescence after W excitation indicates that photoionization had effectively taken place. The absence of the = 1000 run SL band in the photoionizztion cases implies either that: with the (a) The et localized in cavities associated 1000 nm band are bleached off, like the et related to 427
Volume 35, number 3
the 540 nm band, by the W excitation wavelength. This would be uniik-k!r, since krkey et al. [7] have reported that when EtOH glasses were y-irradiated at 77 K and were bleached with Xb d 436 nm at 6 K, an IR band appeared in the absorption spectrum. The reported result indicates that the UV excitation wave= 254 or 313 nm) would not remove the with the Z=IGOO nm band. (b) Or the = 1000 nm band is not related to shallow traps. Teply il l] has reported that the et traps in r-irradiated EtOH glasses at 77 K are practically fully relaxed. length &.,
e; associated
The-absence of,the IR stimulated luminescence .band in photoionization cases and its presence in yradio!ysis cases, therefore, indicate that the Z=1000 nm SL band might be either a dielectron band or an anion band, a radiolyzed pro duct of irradiated EtOH glasses, H20@,0) polycrystalline ices and of irradiated alkaline ices. The former would be improbable because of the low -i-doses used. Besides, the dielectron cen$ers have not been observed in irradiated neutral aqueous solids and alcohol glasses and are probably not stable in these matrices [ 121. The anion band then appears more plausible. The experiment of Namiki ct al. [8] in irradiated EtOH gIasses at 4.K is consistent with our suggestion. By optical absorption rr%a:;urements t!!cse authors observed that with hb = 1000 nm, at 4 K, the 1000 nm band was removed mor: rapidly than the 1 SO0 nm band. The result indicam that two types of traps: physical and chemical, .are present. In fact, if both bands (IO00 and 1500 11rn) were related to shallowly trapped electrons in sinplc-component glzsses, a bleaching photon of 1.:!4 eV (1000 nm) wouldremove the e!ectrons from the mo?e shallow traps (1500 run band) faster than the ones from the less shallow traps (1000 run). The dent observed at the 1000 nm region upon hb = 1000 nm in ::ef. [8] may be interpreted by a transfer of the photodetached electron (e;) from the ICOC) nm traps to the 1500 nm traps. Due to strong overtappiug of these balds, this transfer would, however, not be observable until the 1000 nm hsd been
sufficiently bleached off (curve c ti fig. 2 in ref. [S]). By using X, 5 436 nm i:in the et band) Perkey [7] observed that the I13 band was regenerated. 428
IS September 1975
CHEMICAL PHYSICS LETTERS
et 4.
,&ashimura 1131 also reported that a slight increase of the visible band was observed when bleaching in the IR region. This type of reversible electron transfer or eIectron shutt!ing is typical between chemical traps (10CO nm band) and physical traps (540 nm band) in single-component matrices. It has been suggested previously that the SL band in the IR region observed in y-irradiated HZ0 (DZO) ices might be an anion band [ 141. 4.3. A H-
bmd?
A common anion formed upon radiolysis of EtOH, H,O(D,O) or of aqueous alkaline solution would be either OH- (OD)- or H- (D-). Since the photoionzed solid solutions of tryptophan or of tyrosine in 5 to 10 M of [OH-] do not show any SL band in the IR region [IO], H- would be likely the one. The = 1000 nm SL band might then be correlated with H- (D-). In addit‘ Ion, it is interesting to note that the photodetachment threshold of the z 1000 nm SL band is x0.8 eV, a value which is more appropriate to the electron affi.tity (EA) of the H atom (0.7 eV in the pds phase) than of the OH radical (1.8 eV). In the solid phase one might expect the EA to have a larger value [l]. 4.4. Essq to interpret the experimental results irz y-irradiated pwe solvent glasses with the presence of an =I000 run H- band
The fact that H atoms have not been recorded in y-irradiated EtOH glases even at very low temperafure (4 K) has been puzzling [lS]. The presence of H- in y-irradiated EtOH glasses might accour~t for this abnormality. From the rewlts of ref. [7], after the IR absorption band in r-irradiated EtOH glasses at 4 K was bleached off, with Xb = 500 nm no IR band reappeared, and from the resulti of ref. [8] : the same observations were made at 6 K; with hb G 436 run, however, the IR absorption band was reproduced, one might guess that the blue tail of this IR band (H-) would extend into the visible region and probably ends around 44d nm in solid &OH.
When using Xb > 440 nm, one would photodetach the electrons from the 540 nm (et) traps and simultaneousli from the 1009 nm (H-1 traps:
Volume 35, number 3
\ - ~~zcanm -et
e- ./ m
CHEMICAL PHYSICS LE-I-iERS
‘+C,H,OH;-
/’
CzH50H‘+H ,’
(I)-
/’ C2~,0i+
phosphorescence O: EtOH
at
34_5nm
[Sj
Reaction (I) 116,171 competes with the non-luminescent dissociative attachment [18] : (2) and a
small part of t.he e, may a!so be retrapped. The H atoms, thermal or hot, produced in (1) and (2) react lvith EtOH as well-known [16,17] : H+C,H,OH-+CH:jCHOH+H,.
(3)
At low temperature
(4 K, 6 K) where reaction (3) is slow, part of the H :ltoms newly produced during Ihe bleaching may diffuse and act 2s chemical traps for the oncoming
e,
em :
+ H + H- (1000 nm band) .
(4)
This might expiain Ihe results observed at 4 K by Perkey et al. [7] when using X, G 436 nm (i.e., in the e, band) and might account for the absence of H atoms in irradiated EtOH glasses after optical bleaching even at 4 K [ 151. At 77 K where reactions (2) and (3) overshadow reaction (4), the concentration of H- associated with the =lOOO nm band would probably be too low to be detected by optical absorption. H- mi&t possibly be the “ghost X”, Ha precursor, postulated by Russell and Freeman [ 191, which reacts Gth acid to give H2 but not with NzO to give N2 in the radiolysis of liquid EtOH.
H-(D-) has been observed in electron impact on water vapor experiments [20,21 J. Low e; concentration and higher proton mobility in ices might explain the low intensity of the iR band in the 1~. =f&) spectrum of y-irra 3iated Hz0 or D,O ices [3]. 44.3. MTHF glass es The presence of a H- bnnd in the IR optical absorption spectral region in r-irradiated cY-methyltetrahydrafuran @lTH F) glasses might explain: (i) The differeilce in optical bleaching behatiour of et in r-irradiated MTHF glasses at 77 K observed by
Walker and May 1221 at 1152,694,633
and 442 run
15 September
1975
when using laser pulse at these wavelengths. The authors reported that with the 11.52 nm laser pulse, there was immediate bleaching at 1152 nm then some sLi$C recovery in absorbance over the first few hundred nanoseconds, then slow delayed bieaching. The same results were observed at 694 nrn with the 694 nm laser pulse. At 633 and 442 nm during the pulse, however, there was no bleaching at all at these \vavelengths, then sIow delayed bleaching occurred with a time constant of 0.1 s. In y-irradiated MTHF glasses at 77 K, trapped
electrons have an absorption maximum at = 1200 nm. The observed immediate bleaching at 1152 nm and 694 nm during the laser pulse, might be interpreted as due to the bleaching of et whose absorption band in MTHF glasses overlaps the H- absorption band. The slight recovery of absorbance at these wavelengths over the first few hundred nanoseconds might be
caused by partial iransfer of mobile electrons to the H- band [probably by reaction (4)] and ihe delayed bleaching afterwards might associate with the bleaching of H- . At 633 nm and 442 run, the e, absorption in MTHF glasses at 77 K is low. The only delayed bleaching reported after these wavelength laser pulses 13-21appears to indicate that H- was probably predominantly affected. (ii) The two photon transition process at z9.50 nm region reported by Kevan [23] in y-irradiated MTHF glasses but not in photoionized TMPG-MTHF glasses at low temperature. One might think that the first photon detaches the electron from H- and the second photon excites the mobile electron directly into the conduction band. Hence, a small temperature depecdence observed for this two photon transition [24] might be explained: H- becomes more mobile at 77 K than at 4 EC_ In photoionized TMPD-MTHF, the two photon transition was not observed because there was no H-. The unXor,m decrease of the e, absorption band in photoionized TMPD-MTHF glasses at 25 K upon blcackng with 1064 nm laser light reported by Hager and Wlard [24] was consistent with this suggestion.
H- was originally suggested then later eliminated by WiJJard [25] to account for the decreaSO in [et] in
hydrocxbon
gasses irradiated at 77 K with large Y429
.
CHEMICAL PHYSiCS LETTERS
Volume 35, number 3
in doses e 10zo eV g-l)_ For our own experiments, irradiated 3-methylpentane or methylcyclohexane glasses at 77 K, we observed that the Sue,) band Z= 1600 nm; extending to e-600 nm) can be txrn2x easily bleached off with A,, = 1600 nm at low y-doses eV g-l), iit large r-doses p 102’ eV (lO’QO’9 g-l), with Xb = 1600 nm, however, a weak SL band all:fays remains peaking at 1100 nm (intensity ratio to that of the Suet) band: l/20). This might probably be the (H-j SL band in r-irradiated hydrocarbon
glasses at 77 K. Due to strong overlapping of the dominating et band, the H- band in r-irradiated
alkane glasses could not te observed, until [e;] is sufficiently low, that is at y-doses 2 10” eV g-l 1221. Besides chemical xraps such as solvent radicals [3,25], positive ions produced upon y-irradiation [Xl, H atoms could be other additional scavengers
for electrons during the radiolysis of alkanes.
15 September
1975
References
[l] F. Kiefier and 31. hlagat, Actions &I-I. Biol. Radiations 14 (1970). [2] J. Kroh, W. Roszak and Z. Czerwik, Bull. Acad. Sci. 18 (1970) 699. [3] A. Bernas, J. Blabs, AI. Gauthier, D. Grand and T.B. Truorg, Intern. J. Radiative Phys. Chem. 6 (1974) 401. [4] A. Bernas and T.B. Truong, Chem. Phys. Letters 29 (1974) 585. [5] T.B. Truong and A. Bernas, to be published. [6] A. Bernas, D. Grand and C. Chachaty, J. Chem. Sot. Chem. Commun. (1970) 1667. [ 71 LX. Perkey, Parhataziz and R.R. Hentz, Chem. Phys. Letters 27 (1974) 531. [ 8) A. Namiki, hl. Noda and T. Higashimura, Chem. Phys. Letters 23 (1973) 402. [9] A. Bernas and D. Grand, J. Chim, Phys. 67 (1970) 566. [lo] A. Bernas and T.B. Truong, to be pub&he& [l!] J. Teply, Intern. J. Radiative Phys. Chem. 6 (1974) 379. [ 121 B.G. Ershov and -4.K. Piknev, Advnn. Chem. Ser. 81 (1968)
19.
1131 T. Iiigashimura,
H- formed either by: Q+e;-fI-I;(
during
optical
bleaching
or r-irradiation),
(5)
or
Hi c e- + H, (during y.irradiation),
(6)
might constitute another way for the disparition of Hatoms besides: HtH+H2
(7)
and RH+H+R.+H,.
W
Ann. Rcpt. Rcs. Reactor inst. Kyoto Univ. 6 (1973) 38. [I41 A. Bcrnzs and T.B. Truong, Compt. Rend. Acad. Sci. (Paris) 277B (1973) 391. [I51 L. Kevan, Actions Chim. BioL Radiations i 3 (1969). 1161 C. Chachaty and E. Hayon, J. Chim. Phys. 61 (1964) 111.5. [17! F.S. Dainton, G.A. Salmon and J. Tepiy, Proc. Roy. Sot. A286 (1965) 27; F.S. Ddnton, J.P. Keenc, T.J. Kemp, G.A. Salmon and J. Teply, Proc. Chem. Sot. (1964) 26.5. [la1 M.R. Ronayne, J.P. Guarino and W.H. Hamill, J. Am. Chem. Sot. 84 (1962) 4230. [I91 J.C. RusAl and G.R. Freeman, J. Phys, Chcm. 72 (1968) 816. [201 E.N. J&settre and W.M. XUO, J. Chem. Phys. 61 (1974) 1703. [21 I S. Trajmu and R.HalI, J. Phys. B7 (1964) L458. WI D.C. Walker and R. May, Intern. J. Radiative Phys. Chem. 6 (i974) 345. J. Radktikz Phys. Chem. 6 (1974) [231 5. I&van, Intern.
297.
Acknowledgement The author would lik: to thank Dr. A. Bernas for a critical reading of the manuscript. The help in preparing the samples from Mr. A. Petit is also appreciated.
[241 S.L. Hager and J.E.‘iWlard, I. Chem. Phys. 61 (1974) 3244. [251 J.E. Willard, Ictern. J. Radiative Phys. Chem. 6 (1974) 32.5.