On the existence of H- in Y -irradiated matrices at 77 K

A. BERNAS A N D T. B. TRUONG E. R. CIVRS 98, Unii~ersi fc:Pnri~-S~id, 91405 Orsay, Frctrlce

Received September 27. 1976

A. B E R ~ A S and T. B. T~uoxc. Can. J. Chem. 55.2044 (1977) A band peaking around 1050 nm in the stimulated neutralization luminescence spectrum of

y-irradiated glassy EtOH, pure crystalline and alkaline glassy ices has recently been attributed to H- . Further results on EtOH glass and on EtOH-H,O mixtures are now presented which support our previous interpretation.

A shuttle of the electron from the presumed H to a matrix trapped state and back to H with restoration of the 1050 nm band can be induced in, and only in, alcohol-water mixtures where it has been shown that, contrary to either of the two pure solvents, H atoms can be trapped (H,). Further, the intensity of the regenerated stimulated band is found to be lnaxirnum for the mixture con~position which leads to the highest H, yield.

It is noted that the red limit of the H- stimulated band, which corresponds to a photo- detachment energy threshold, is close to the electron affinity value of the isolated H atom. This observation seems to imply that, contrary to expectation, the solvation energy of H - in these matrices is quite low.

A. BERNAS et T. B. T R L O ~ G . Can. J . Chem. 55. 2044 (1977). La glace pure cristallisee, les glaces alcalines vitreuses, I'ethanol vitreux irradies aux Ri. B

77 K donnent lie~l a une luminescence de neutralisation dont le spectre d'excitation presente une bande centrte B 1050 nm. I1 a etC recemment suggere que celle-ci Ctait associte B H - . De nou- veaux resultats relatifs B EtOH vitreux et aux melanges EtOH-H20 sont maintenant prtsentes qui confirment cette interpretation.

I1 est en effet observe qu'un transfert reversible des electrons de I'espece X- vers des pieges physiques puis vers X avec reconstitution de la bande X- a 1050 nm n'est possible que dans les melanges EtOH-H,O ou les atomes H sont stabilists (H,). L'intensite de la bande 1050 nm ainsi regeneree est trouvee lnaximale pour la composition du melange qui conduit a un rende- ment maximum en H,.

On note enfin que la limite vers le rouge de la bande de stimulation de H - qui correspond B une tnergie seuil de photodetachment est tres voisine de la valeur de I'affinite electronique de H gazeux. Ceci semble impliquer que dans les matrices considCrees l'energie de solvatation de H - est tres faible.

Introduction It is well known that after y-irradiation of

rigid matrices at low temperatures and stabiliza- tion of neutral and charged intermediates, solvated electrons may be detrapped or bleached by a suitable optical excitation. A deferred luminescence designated as 'stimulated' lumines- cence (SL) is then generally associated with the subsequent cation-electron neutralizatioii (1, 2).

The emission spectrum, I,, = f(h,,), obtained upon bleaching the trapped electron at constant ~iavelength h, and recording the luminescence at variable wavelength h ,,,,,,,,, may give infor- mation on the neutralization mechanism, dis- sociative or not, and on the emitting electronic state (3).

On the other hand, the luminescence excita- tion spectrum or 'stimulation' spectrum, giving the neutralization luminescence intensity as a

function of the bleaching wavelength, I,, = f(?,,), characterizes the negative species, whether anions or solvated electrons (2-5).

This technique has recently been applied to y-irradiated glassy ethanol and the results have led to the suggestion that H- was present (6 ) . The present study deals with y-irradiated ethanol and ethanol-water mixtures at 77 K where hydrogen atoms are known to be stabilized, although trapped H ( H , ) is not observed in either pure component (7).

Experimental Results and Discussion The experimental set-up has been described

elsewhere (4, 5) . It mainly consists of a slightly modified commercial spectrofluorimeter in which the xenon source and excitation monochromator are used to release the trapped electrons in the pre-irradiated sample. The neutralization lumin-

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BERNAS AND TRUOKG 2045

escence is simultaneously recorded by a RCA IP that there would be no emitter as such; the 28 photomultiplier through the analyzing mono- luminescence would be a property of the ion chromator. The stimulation spectra have been pair. corrected for the trapped electron consumption For alcohol, it can only be stated that neither under successive bleachings and for the varia- alcohol impurities (curve B) nor stable radio- tion of the excitation light flux with bleaching lysis products (curve C) are responsible for the wavelength A,. 345 nm emission band.

EtOH Emission Spectrum

For pure ethanol glasses y-irradiated at 77 K, the observed neutralization emission spectrum is given in Fig. 1 , curve A . The same emission spectrum peaking a t 345 nm is recorded, what- ever the bleaching wavelength: 500 or 1000 nm.

The nature of the emissive center has been much debated for irradiated alkane glasses, with n o conclusive assignment, however, and the emitter is also unidentified in the alcohol case. For the former, Brocklehurst (8) has recently proposed a radiative tunneling mechanism so

20pntensity (arbitrary units)

A,, (nm) FIG. I . Neutralization emission spectrum in y-irradi-

ated EtOH glass at 77 K. (A) Stimulated emission spec- trum, Is,, = f(h,,) of y-irradiated EtOH glasses, (10r9eV g-I), upon bleaching with h, = 1000 nm. (B) Emission spectrum of unpurified EtOH glass, upon excitation at he,, = 254 nm. (C) Emission spectrum of the y-irradiated sample ( loL9 eV ggl) subjected to one melting-freezing cycle, and excited with he,, = 254 nm.

Stiviulution Spectrun? However, whatever the origin of the emission,

the luminescence excitation or stimulation spectrum provides an electron photodetachment efficiency curve. If 3 L a n a l Y s i s is fixed at 345 nm, the stimulation spectrum shown in Fig. 2 is obtained. The visible region of the spectrum appears quite slmilar to the previously reported (9, 10) bleach- ing efficiency curves relating the decrease in trapped electron optical density to the bleaching wavelength A,. An extra band, absent in the bleaching curves is, however, present in the stimulation spectrum peaking at about 1050 nm with a low energy threshold at about 1500 nm (0.86 eV). The intensity of the 1050 nm band decreases as the y-dose increases as illustrated in Fig. 3.

An analogous band appears in the stimulation spectra of pure (11) or alkaline (12) ices. If present in the stimulation spectra of irradiated alkane glasses, it would be masked (6) by the

FIG. 2. Neutralization stimulation spectrum in y-irrad- iated EtOH at 77 K. ( A ) Stimulation spectrum I,, = f(?.,), ha, = 345 nm. (B) Bleaching efficiency curve (- D,,,) = f(h,) (9). (C) Absorption spectrum of sol- vated electrons in y-irradiated EtOH glass (9).

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CAK. J. CHEM. VOL. 5 5 . 1977

FIG. 3. Intensity of stimulated luminescence in y-irradiated EtOH glass cs, y-dose; I , , , = 345 nm, i.., = 1000 nm.

broad trapped electron band peaking at 1600 nm. It has been observed (6), however, that the bleaching rate at 1600 nm for MCH or 3-MP glasses is much lower after large y-doses (> 10'' eV g-I), that is, when the trapped electron contribution is lower. Such a difference in the bleaching kinetics between physical and chemical electron traps has been pointed out before (13, 4). This observation would favour the con- clusion that the 1050 nm band originates from an anion.

Absorption bands in the same spectral region have been reported previously.

In the case of alkaline aqueous glasses, the 1000 nm band has been attributed to dielectrons (14). In the case of ethanol irradiated and investigated at 4 (15) or 6 K (lo), the is band apparently originates from electrons located in shallow ~jells, non-relaxed traps. For n-pro- pan01 and higher aliphatic alcohols the shoulder observed in the near is region would correspond to traps formed, at least partly, by alkyl groups (16).

None of these interpretations can apply to our experimental findings, however. A dielectron attribution cannot be considered since the 1050 nm band appears at low doses for irradiated ices (1 1) and since its intensity decreases as the y-dose increases in the ethanol case (Fig. 3). As to the shallow traps, they are known to be stabilized at 4-6 K but to become fully relaxed as the tenlperature rises to 77 K (17) with a correlative shift of the absorption band maxi- mum to higher energies. Besides, the 1050 nm band does not exist in the SL spectrum of photoionized tryptophane dispersed in ices or ethanol glasses (6). Electrons which would be

solvated not only by OH dipoles but also by alkyl groupings might exist in alcohols but not in pure or alkaline ices. Again, we are led to suggest that the 1050 nm band is associated with an anion X-.

OH radicals, and possibly OH-, are stabilized in pure radiolyzed ice, 0- in alkaline ices, but the most plausible radiolysis product X- , common to alcohols and ices, seems to be H- .

EtOH-H20 Mixtures It is well known that no H atom signals have

been recorded through esr in alcoholic matrices neither at 77 nor even at 4 K, even though much indirect evidence exists that hydrogen atoms are produced during radiolysis. In pure ice, H atoms are stable at 4 K but begin to diffuse and com- bine in the temperature range 20-50 K.

On the other hand, Hase and Kevan (7a) and Chandra (7b) have identified trapped hydrogen atoms through the characteristic esr doublet signal of H in irradiated MeOH-H,O, EtOH- H,O, and PrOH-H,O mixtures at 77 K. Since the H, yields as a function of the mixture com- position show a striking correlation with the excess enthalpy of mixing of the alcohol-water mixtures, it was considered that both effects are related to alcohol-water complex formation, and that the simplest trapping site structure was a hydrogen bonded trimer of one alcohol and two water molecules (7u).

Neutralization luminescence has now been examined for these systems and the following observations have been made.

Errlission Spectrum For the EtOH-H,O mixtures (10 to 50 m o l z

of EtOH), the stinlulated emission spectrum

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gives a 3,,,, at 360-365 nm under bleaching at 1000, 390, or 254 nm. This emission is red- shifted as compared with the stimulated emission of pure EtOH (h,,, = 345 nm) and blue-shifted with respect to the main neutralization lumines- cence band in pure irradiated ice (A,,, = 380 nm).

It is not clear at the present time whether the 365 nm emission is an envelope of the two pure solvents' emission bands or is due to a different emissive entity.

Stimulation Spectrum The stimulation spectrum has the same shape

as the EtOH stiinulation spectrum (Curve A, Fig. 2) but, in the irradiated EtOH-H20 mix- tures, the 1050 nm band appears more intense than for either of the two pure solvents.

The 1050 nm band due to X can be com- pletely bleached off under h, = 1000 nm. Then, by exciting the sample with a light frequency which is absorbed by the solvated electrons and not by X- , i.e. at h, = 400 nm (6) the 1050 nm band is partially regenerated. This restoration of the X band is observed in the EtOH-H,O mixtures but in neither one of the pure solvents. That is, the X- band can be reproduced only in the systems where H atoms are stabilized, in which case one can induce the reversible electron transfer:

[ I1 H e,,,, - f hv + em- + X,-

[21 XI- i- hv -t e,,,,-

Moreover, as shown in Fig. 4, the intensity of the regenerated X - band appears maximum for a mixture containing 20 m o l z EtOH. This is precisely the mixture composition which, from Hase and Kevan experiments (7a), leads to the maximunl concentration of trapped hydrogen atoms, following y-radiolysis or after a sub- sequent e,,,,- optical bleaching.

Similarly, the intensity of the regenerated X- band increases with increasing y-dose within a range where H, concentration was found to be linear with dose: curve A (y-dose = 0.6 Mrad) as compared with curve B (y-dose = 0.32 Mrad).

The restored X- band appears more intense for h, = 254 nm rather than h, = 390 nm which may reflect a higher e,,,,- photobleaching efficiency at shorter wavelengths (curves B and el.

These qualitative and quantitative features

VD TRUONG

t I" I" " , )

0.1 0.5 1.0 X E t o H

FIG. 4. Intensity o f the regenerated 1050 nni band as. EtOH mole fraction in EtOH-H,O mixtures: ( A ) y-dose = 0.6 Mrad, i,, = 254 n m , (B) y-dose = 0.32 Mrad, h, = 254 n m , (C) y-dose = 0.32 Mrad, 7. = 390 nm.

indicate that X- anion is effectively the hydride ion, as previously suggested (6).

It is also worth mentioning that H - has been detected in solar atmosphere (18) and that its calculated and observed absorption spectrum appears at the same spectral position, as illus- trated in Fig. 5.

The decrease of the H stimulated band with increasing y-dose for pure EtOH may be com- pared with the observed decrease with dose of the molecular hydrogen yield G(H2). It may be similarly explained by a competition between the H forming reactions 3 and 4

and the electron attachment on acetaldehyde, which i5 a good electron scavenger and whose concentration increases with dose 119).

A competition may also intervene between reactions 5 and 6:

where R' is a radiation produced solvent radical. Various modes for H - formation may be

envisaged depending on the matrix:

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2048 C,ZN. .I. CHEM. VOL. 5. 1977

1 I,, ( 0 . ~ 1

FIG. 5. Absorption spectrum of gaseous H-: (A) Stimulation spectrum ILs = f(Iib) of y-irradiated EtOH glass, y-dose = 1019 eV g-I, i,,, = 345 nm, (B) Observed absorption spectrum of H - (18), (C) Calculated absorp- tion spectrum of H- (18).

(n) In water vapor. the dissociative electron attachment [7] is well established (20)

Even though at higher pressures ion-molecule reaction 8 .

[81 H- + H z 0 -, OH- + HZ

is known to occur, reaction 7 may be operative and H- might be solvated and stabilized in condensed phase.

(b) In the EtOH-water mixtures, electron attachment may clearly occur on preformed, trapped hydrogen atonis

besides their possible for~nation during radiolysis through reaction 10.

(c) In pure radiolyzed solid alcohol, where hydrogen atoms are not trapped, the latter may diffuse toward the solvated immobile electrons and become trapped as H,- :

Reaction 10 would thus provide another path for the decay of H atoms, besides the conibina- tion reaction 11

and the abstraction reaction 12:

Shirom and Willard (21) to account for the decrease in solvated electron concentration with increasing dose in hydrocarbon glasses. It was shown later (22) that reaction 10 was not responsible for the observed decrease in G(e,,,,-); there is no indication, however, that reaction 10 does not take place in hydrocarbon glasses or other radiolyzed systems.

As to the occurrence of reaction 9, it is not easy to demonstrate. Electron spin resonance analyses of the y-irradiated EtOH-H,O mixtures have shown (7a) that, upon bleaching the sol- vated electrons, the H atom concentration in- creases due to H production through H,Ot (mostly) and C,H,OH,+ neutralization. Such an increase apparently overcomes the decrease which would be associated with electron attach- ment on H. The fact that the decrease in e,- is found to exceed the correlative increase in H (70) indicates that some of the bleached elec- trons do not recombine with positive holes. This may be due to reaction 9 but also to electron attachment on other radiolysis products, attach- ment which, if dissociative. would not give rise to H.

Conclusion We wish to conclude with two remarks. The low energy limit of the H- stimulated

band, which corresponds to a photodetachment energy threshold, appears very close to the value of the electron affinity of the isolated H atom (0.7-0.8 eV). This observation is contrary to cxpectation. It would seein to imply that the inillueilce of the matrix molecules, i .e . the solva- tion energy of H- in these matrices, is quite low. As pointed out to us,' this could possibly be correlated to the helium-like, close-shell con- figuration of the hydride ion.

Finally, we want to stress again that recording the stimulation spectrum of the neutralization luminescence is a detection technique which is both sensitive and specific of negatively charged species. It may thus supplement optical absorp- tion spectroscopy when the sensitivity of the latter is too low or when cations and anions have indistinguishable spectra. It may also replace esr spectroscopy in the search for nonparamag- netic anions.

1. E. DOLAN and A. C. ALBKECHT. J . Chem. Phys. 37. 1149 (1962).

2. A. DEROULEDE. J . Lumin. 3, 302 (1971).

Reaction 10 has been previously suggested by 'L. G. Christophorou, private communication.

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BERNAS AND TRUOKG 2049

A. BERNAS, J. BLAIS, M. GAUTHIER, D. GRAND, and T. B. TRUONG. Int. J . Radiat. Phys. Chem. 6, 401 (1974). T . B. TRUONG and A. BERNAS. J . Phys Chem. 76, 3894 (1972). T. B. TRUONG, A. BERNAS, and J. RONCIN. J . Phys. Chem. 78,867 (1974). T. B. TRUONG. Chem. Phys. Lett. 35.426 (1975). ( a ) H. HASE and L. KEVAN. J. Phys. Chem. 74, 3355 (1970); (b) R. CHANDRA. Int. J. Radiat. Biol. 17, 497 (1970). B. BROCKLEHURST. Chem. Phys. Lett. 39,61 (1976). A. BERNAS, D. GRAND, and C. CHACHATY. J . Chem. Soc. Chem. Commun. 1667 (1970). L . M. PERKEY, FARHATAZIZ. and R. R. H E N T ~ . Chem. Phys. Lett. 27,531 (1974). A. BERNAS and T. B. TRUONG. C. R. Acad. Sci. Ser. B. 277.391 (1973). T. B. TRUONG. J . Chem. Phys. In press. K. FUNABASHI, C. HEBERT, and J . L. MAGEE. J . Phys. Chem. 75, 3221 (1971). L. KEVAN. D. RENNEKE, and R. J . FRIAUF. Solid State Commun. 6,469 (1968). A. NAMIKI, M. NODA, and T. HIGASH~MURA. Chem. Phys. Lett. 23, 402(1973). T. SHIDA, S. IWATA, and T. WATANABE. J . Phys. Chem. 76,3683 (1972). J. TEPLY. Int. J. Radiat. Phys. Chem. 6, 379(1974). H. S. W. MASSEY. Negative ions. Cambridge Univer- sity Press, London. 1950. p. 128. G. R. FREEMAN. In Actions chimiques et biologiques des radiations. 14e serie. Masson Ed. 1970. p. 128. R. N . C O M P ~ O N and L. G. CHRISTOPHOROU. Phys. Rev. 154, 110 (1967). M. SHIROM and J . E . WILLARD. J . Am. Chem. Soc. 90,2184 (1968). A. ECKSTROM, R. SUENRAM, and J . E. WILLARD. J . Phys. Chem. 74, 1888 (1970).

Discussion

R. S. Dixon: H is thought to react rapidly with H,O in the gas and liquid phases. Would you expect reaction with neighboring water molecules in the glass?

A. Bernas: I agree that at high pressures the ion-molecule reaction H- + H,O -. O H + H, is known to occur. However, H- may be formed in a trapped state through

+ e , " ~ , * Hcapped .

N. K. Kestner: I am also concerned that the spectral shifts of the H spectrum are so small in the glass: H- is a rather large system, compared to H. H- is only stable relative to H + e- by 0.75 eV while the correlation energy is over 1 eV. In order for H- to be stable the size of the orbitals in H expand. Thus I might expect larger spectral shifts for H- than for H when present in condensed phases.

A. Bernas: As I just mentioned we were also puzzled that the photodetachment threshold in the glass appears close to the electron affinity of H in the gas, and so far we have no explanation to offer. As to the size of the H orbitals permit- ting H- stability, it seems difficult to say anything in the case of condensed media.

R. Catterall: When the solvated electrons in r-irradiated water-alcohol solutions are photobleached, the esr signal from e , , , , disappears. Does the H atom esr signal also decrease?

A . Bernas: The experiment has been done but the data are not conclusive with regard to H formation. ( a ) The initial (i.e. before e,,,,- bleaching) H concentration is low as compared to that of e , , , , . (b) Upon e,,,,- bleaching vari- ous reactions will modify the H concentration in opposite directions: neutralization of H,O' or attachment on ethanol molecules will increase [HI whereas electron at- tachment on H will decrease it.

U. Schindewolf: Our attempts to dissolve alkali hydrides, e . n . in THF with the aid of crown-compounds, to complex tbe alkali ion failed. We hoped in this-way to get H- jons

L. G. Christophorou: Could the fact that H- is a closed- into solution, Has H- even been observed i n liquid solution shell system explain the absence of a profound medium Na-, K-, etc, have been? effect (on the photodetachment threshold)?

A. Bernas: Not to my knowledge, even though it has been A. Bernas: This is indeed a very interesting comment and suggested many times, in particular a long time ago by suggestion but before drawing such a conclusion I would Platzman, prefer that our experimental work be extended to other anions, r .g . CI

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