D. Huppert, P. M. Rentzepis, and W. S.Struve
2850
Picosecond Dynamics of Localized Electrons in Metal-Ammonia and Metal-Methylamine Solutions D. Huppert, P. M. Rentrepis; Bell Laboratories, Murray Hill, New Jersey 07974
and W. S. Struve Depatfment of Chemistry, iowa State Universlty, Ames, lowa 50010
(Received September 2, 1975)
Publication costs asslsfed by Bell Laboratories
Experimental studies are presented here for time and wavelength resolved optical bleaching of the 1.5-p solvated electron absorption band in liquid ammonia by 1.06-p single pulses of -6 psec duration, and for electron localization in methylamine following photoionization of the M- species in sodium-methylamine solutions by 530-nm picosecond pulses. Bleaching of the solvated electron band in methylamine by 1.06-p pumping pulses was also studied. In both ammonia and methylamine solutions, the solvated electron absorption band is found to be homogeneously bleached between -800 and 1100 nm, and the relaxation time of the 1.06-p populated excited state of the localized electron is -2 X sec. The electron localization process in methylamine occurs within 5 psec of creation of the quasifree electron at temperatures between -40 and -8OOC.
Introduction A great deal of effort has been studied toward understanding the physical properties of dilute (510-3 M ) metal-ammonia and metal-methylamine so1utions.l Diverse experimental techniques, including optical absorption spectroscopy,2 calorimetric3 and conductance studies: EPR and magnetic susceptibility determination^,^ and NMR Knight shift measurements,6 have shaped some consensus concerning the structure of such systems. For sufficiently high temperatures and low concentrations, alkali metal-ammonia solutions are considered to contain only independently ammoniated alkali cations and electrons. The latter account for the solutions’ broad 1.5-p optical absorption band,2 which undergoes blue shifts with decreasing temperature, and varies little for different alkali metals. This spectrum is believed to arise primarily from the 1s 2p transition between bound states of the trapped electron, though higher transitions (to additional bound states converging to a free-electron continuum7) may well cause the band’s short-wavelength “tail” extending well into the visible. Electronic structure calculations8 derived from a simple polaron model for the lowest bound states yield excellent agreement with the spectral transition energy and oscillatpr strength (6 -5 X lo4 M-l cm-l at 1.5 h). By comparison, dilute metal-amine solutions exhibit optical and EPR spectra which can only be rationalized by the existence of at least three specie^.^,^^ The solvated electron is believed to be responsible for the single sharp EPR band, as well as for the infrared absorption band (1.3-p band maximum, Av -6000 cm-l) observed in methylamine-, ethylamine-, and EDA-metal solutions. As in the metal-ammonia solutions, this band’s position and shape are independent of the alkali metal. Secondly, a diamagnetic speciedl is responsible for the optical absorption band located in the red and near-infrared region. Unlike the solvated electron band, this band depends markedly on the alkali metal, so that it has been assigned to the alkali metal anion M-. The band maxima for Na- and possibly Li- are situated at 660 nm, while those for K-, Rb-, and
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The Journal of Physical Chemistry, Voi. 79, No. 26, 1975
Cs- are displaced to 850, 930, and 1030 nm, respectively. Finally, a monomeric M species is likely to be the origin of the hyperfine structure observed in the EPR spectra, though no corresponding optical absorption spectrum has been identified. By contrast, virtually nothing is known about the relaxation dynamics of either quasifree or excited trapped electrons in liquid ammonia or amine solutions. Because of its comparative simplicity in molecular and liquid structure (especially in comparison to strongly hydrogen-bonding solvents), liquid ammonia provides an attractive prototype for phenomenological,12SCF,13and pseudopotentiall4 calculations of excess electron distributions in polar solvents. The ammoniated electron’s predicted emission band maximum, originating from a vertical transition between the relaxed 2p state and the 1s state, is drastically red-shifted to -0.5 eV from the 0.9-eV absorption band owing to the diffuseness of the equilibrium 2p charge density.15 No such fluorescence has ever been observed in metal ammonia solutions, indicating that nonradiative relaxation of the bound 2p state occurs on a time scale at least two orders of magnitude faster than the pure radiative lifetime T?. The latter has been estimated from the 2p 1s transition moment to be T~ l nsec, so that the 2p state decays within 10 psec or less. Of equal interest is the question of whether the 1.5-11 band (as well as the 1.3-p band in the metal-amine solutions) arises from homogeneous broadening of the trapped electron’s lowest allowed electronic transition. If so, the entire spectrum should be proportionately bleached by narrow band pumping (Av 100 cm-l at 9431 cm-l in the present experiments). Evidence for wavelength-dependent bleaching (“hole burning”) has recently been cited1’ as proof of inhomogeneous broadening of trapped electron absorption band in MTHF and 3-methylpentane glasses, indicating those spectra are broadened by local variations in cavity radii or dielectric constant. The following alternative processes may be visualized for the nonradiative relaxation of electronically excited trapped electrons. (a) The solvated electron absorption
-
-
-
Picosecond Dynamics of Localized Electrons
2851
1060 nm
-L
CONTINUUM (750-1100nm)
A
Figure 1. Schematic representation of the experimental system. The components are: (1) Nd3+ glass oscillator with cavity mirrors CI, Cz: (2) saturable dye absorber cell; (3) Pockels cell positioned between crossed Glan polarlzers PI, Pz; (4) spark gap: (5) Nd3+ glass amplifiers; (6) translatable prism used to generate variable delay; (7) broad band continuum cell containing CCI4: (8)stepped-delay transmission echelon: (9) sample cell; (10) monochromator; (11) silicon vidicon optical data digitizer; (12) Nova computer: (13) graphics terminal. Mirrors are denoted by M, beam splitter B, optical filter F.
-
with Na metal. After distillation of clean solvent into the band originates from the bound-bound 1s 2p vertical sample vessel and addition of -50 mg of Na metal (Merck, transition. The excited state relaxes to an equilibrium 2p vacuum distilled), additional degassing and baking of the configuration which then crosses to the ground state.15 Invessel walls were applied before the sample was sealed off. volvement of a long-lived 2s excited state appears to be Analogous procedures were followed in the preparation of ruled out here, because the 2s state is of higher energy than Na-methylamine solutions. Care was taken to ensure that the 2s state and cannot be populated without appreciable sample absorption spectra conformed to previously reactivation energy. (b) The Is 2p transition is followed by ported bands2 for M- and the solvated electron. The use of thermal ionization of the excited state, yielding a quasifreels electron which is trapped to form the 1s ground 2-mm absorption cells reduced the optical density of solvent vibrational overtone bands21 at 1.06 1to 50.05, constate. This electron localization process has attracted considerable study in liquid waterls and a l c o h ~ l sand , ~ ~can ~ ~ ~ siderably less than the picosecond optical density changes be described in terms of two consecutive processes: an ulreported in this work. A 7-cm diameter optical Dewar furtrafast relaxation process, induced by nonadiabatic counished with -2-cm diameter Pyrex windows housed the pling between the electron's quasifree and 1s ground states, sample, which was flushed by Nz gas streams at -78 and producing the latter in a nonequilibrium solvent configura-195OC; sample temperatures were monitored by a Cution, and subsequent solvent relaxation, occurring on the constantan thermocouple suspended -5 mm from the samtime scale of dielectric relaxation. (c) The quasifree elecple cell. Na-NH3 solutions had an optical density of -1.0tron is produced in a bound-continuum transition, and is 1.3, and temperatures were maintained a t -60 to -75OC. then trapped to form the localized ground state. Na-methylamine solutions were adjusted to have densities of -0.15-0.4 at 530 nm, and were used from -40 to -8OOC. In the present work, we report picosecond bleaching studies of dilute (-2 X M) Na-NH3 solutions pumped Both types of solution were quite stable at liquid N2 temat 1.06 1. Pertinent information has been extracted here perature, though the concentrations of dilute Na-methylconcerning the line broadening and radiationless relaxation amine solutions declined by -20% after several hours at of the solvated electron in liquid "3. In addition, we have -8OOC. Sample solutions were replaced within 2 weeks of studied the dynamics of electron localization in Na-mepreparation. thylamine by ejecting an electron with a single 530-nm piOptical Arrangement. Figure 1 summarizes the optical cosecond pulse overlapping the 660-nm Na- absorption arrangement providing the-picosecond bleaching and interrogating pulses in the Na-NH3 experiments. A Nd3+ glass band. Following the subsequent localization of the solvated oscillator (Brewster/Brewster rod dimensions -7 in. long X electron in methylamine, the optical bleaching of the 1.3-1 0.5 in. diameter) was mode-locked with Eastman Kodak band by a 1.06-1 pulse was studied with a view to interpret9860 saturable dye in a 1-cm cell, yielding 1.06-1 pulse ing the band's broadening mechanism as well as the relaxatrains with average width -8 psec21 and separation -7 tion behavior of the excited trapped electron. nsec. Mode locking was monitored with an ITT F4000 S1 Experimental Section fast photodiode and Tektronix 519 oscilloscope. Single Sample Preparation. Concentrated 3-ml samples of Napulse extraction22was achieved with a Pockels cell situated NH3 were prepared in sealed glass vessels comprising vacbetween cross Glan polarizers. A 21-kV spark gap triggered uum-line optical cells with side arm extensions. Matheson by the first few rejected mode-locked pulses supplied a anhydrous NH3 gas was purified by several liquid N2 -4-nsec duration half-wave voltage to the Pockels cell. freeze-pump-thaw cycles before purification by reaction Amplification of the transmitted 1.06-p single pulse was
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The Journal of Physical Chemistry, Vol. 79, No. 26, 1975
D. Huppert, P. M. Rentzepis, and W.
2852
provided by two Brewster/Brewster Nd3+ glass amplifiers with a total measured gain of -50. A multilayer dielectric beam splitter reflected -45% of the amplified 1.06 11 pulse for sample excitation. The remainder was focussed into a 20-cm path length cell containing CC14, where self-phase modulationz3 of the laser fundamental and stimulated Stokes Raman band produced a broad band continuum pulse24 of duration comparable to the 1.06-11 pulse. This n
U
T psec ( - 6 psec /SEGMENTS)
S.Struve
was split into an interrogating train of 10 pulses separated by 3.3, 6.7, or 20 psec using an appropriate transmission echelon. The echelon train was focussed into a sample cell. After passing through a McPherson 0.3-11 monochromator (53 A/mm dispersion), each of the interrogating pulses was sharply imaged onto a distinct spatial regionz5 of an RCA 4532 silicon vidicon. The vidicon scan was controlled over a programmable two-dimensional grid by a Nova 1230 computer (Data General Corp., interfacing electronics provided by EMR Photoelectric). A liquid Nz vidicon cooler (Products for Research, Danvers, Mass.) maintained the photocathode temperature at -9OOC. Interrogating light intensities from each laser shot were stored by position (256 X 28 resolvable points were read from a thin, rectangular portion of the entire vidicon target for an accelerated readout) onto a moving-head disk file for further processing. These signals were integrated along the axis normal to the time coordinate to produce one-dimensional plots (Figure 2) on a computer graphic terminal. Here, each sharply defined peak corresponds to a particular pulse in the interrogating train. The principal modification of this apparatus for the Namethylamine experiments (Figure 3) consists in that two bleaching pulses were directed into the sample rather than one. The first of these, a 530-nm single pulse generated by a second harmonic generator from the 1-06-11fundamental in a 1-in.3 KDP crystal, photoionized the Na- species to form an ejected electron which became solvated. Approximately 60 psec later, this was followed by a 1.06-11 single pulse which bleached the solvated electron absorption spectrum. When interrogating pulses were required to monitor bleaching of the Na- absorption band at 600-660 nm, 1-octanol was substituted for CCld as the continuum liquid. The synchronization of arrival of the 1.06-11 bleaching pulse and the interrogating pulses in the Na-NH3 experiments was verified by placing an Eastman Kodak 9860 dye solution (D 1.0 at 1.06 11) in place of the sample. Quantitative bleaching of the saturable dye, lasting -8-10 psec at 1.06 w, was achieved with unamplified 1.06-11 bleaching pulses (estimated energy -2 mJ/pulse), but attenuation of these pumping pulses with neutral density filters proportionately diminished the bleaching optical density change. As the extinction coefficient (a 4 X lo4 M-l cm-l) and re-
-
psec ( - 6 psec
SEGMENTS)
Figure 2. Histogram of echelon pulses traverslng through the ND3Na 2-mm cell. Intersegment separatlon 6 psec, wavelength 1100 nm, I designates relative transmitted Intensity; (a) transmitted echelon segments without excltatlon; (b) same as (a) with excltatlon occurring at t = 0 segment.
GRAPH I C s TERMINAL
C4 COMPUTER
HV
A
530 nm ( C G E N E R A T I O N )
A to60 nm (esoLv BLEACHING1 A
SILICON VIDICON
MONOCHROMATOR
0
600 n m - 1120 nm) CONTINUUM ( I N T E R R O G A T I N G )
Figure 3. Schematlc representatlon of the experimental system. The components are: (1) Nd3+ glass oscillator with cavlty mirrors Ci, Ca; (2) saturable dye absorber cell: (3) Pockels cell posltioned between crossed Glan polarizers PI, Pz; (4) spark gap: (5)Nd3+ glass amplifiers; (6) second harmonic generator: (7) second harmonic generator; (8) broad band continuum cell containing CCId or 1-octanol: (9) stepped-delay transmission echelon: (10) sample cell; (11) monochromator; (12) slllcon vidicon optical data digitizer; (13) Nova computer; (14) graphics termlnal. Mirrors are denoted by M, beam splitter B, optical flber F. The Journal of fhyslcal Chemistry, Vol. 79, No. 26, 1975
2853
Picosecond Dynamics of Localized Electrons
r
io6onm
-.jI
530 nrn
a 1060 nm
A x.940 nrn T:-60aC
f ( p s e c ) 2opsec/seg J
Figure 4. Display of the synchronization of the single 530-nm excitation and 1060-nm bleaching pulses with the 1000-nm interrogating 10 echelon light segments. It is seen that each segment is 20 psec in width, and the 530 nm is coincident with the fourth echelon segment while the 1060 nm arrives three segments later equivalent to 60 psec after the 530 nm-pulse.
laxation time of the dye are independently known, these auxiliary experiments served to calibrate bleaching intensities in Na-"3. In -the Na-methylamine studies, synchronization of the 530-nm and 1.06-11.pulses and the t = 0 interrogating pulse was determined with a CS2 optical shutter.26 The CS2 cell was subsequently replaced by the methylamine sample at exactly the same position. Owing to the effects of wavelength dispersion in the continuum liquid, this synchronization was checked at all wavelengths used. Figure 4 shows the timing of 1000-nm echelon interrogating pulses with the 530-nm and 1.06-p pumping pulses. It is evident here that the delay between excitation pulses was adjusted at 60 psec, in other cases, the pulse delay was varied between 40 to 80 psec depending on the species studied. The 1.06-p pumping pulse energy in both experiments was typically -30 f 10 mJ, while the 530-nm photoionization pulse contained -10 mJ. The optical density of the Na-NH3 solution was maintained -1.3 at 1060 nm while the Na-methylamine was 0.3 at 530 nm. A typical optical density change at 1000 nm was -0.8 for the formation of the solvated electron band in methylamine. Much smaller density changes were observed for bleaching of the solvated electron band in Na-NH3 and Na-methylamine, however, these were symptomatic of rapid repopulation of the trapped electron's ground state (vide infra). Experimental optical density changes AD were determined by firing laser shots alternatively with and without the 530-nm and/or 1.06-p pulse entering the sample cell. Values for AD were computed via AD = log (Iw/In), where I* and In denote the intensity of the particular interrogating pulse in the presence and absence of the pumping pulse(s), respectively.
Results Ammonia. Bleaching of Na-NH3 and Na-ND3 solutions by 30 f 10 mJ, 1.06-p pumping pulses was observed with 10-nm resolution between 850 and 1100 nm with echelon pulse spacings of 20, 6.7, and 3.3 psec. Reduction of raw data similar to that presented in Figure 2 results in the typical results depicted as optical density changes for 940-, 1040-, and 1100-nm interrogation in Figures 5 and 6. When the 20-psec echelon was used, the bleaching and recovery of the solvated electron band occurred within one echelon segment. It is apparent from the data of Figure 7 that the optical density changes AD follow qualitatively the same
-80
0
-40
80
40
r (Dsec) 0.1
b
0
7 -
--u-
NH3/N,,
A=io40 nrn
0
0
T=-60'C
4
-0 1
-80
,
1
-40
0
1
40
I
80
r (psec) Fiaure 5. Time-resolved bleachina and recoverv of the solvated electron absorption band of the soiium ammonia solution at (a) 940 and (b) 1040 nm. As shown kinetic data are achieved by 2Oipsec echelon segments. The excitation was performed by a single 1 . 0 6 ~ pulse generated by a mode locked Nd3+ glass laser.
wavelength dependence between 850 and 1100 nm as the absorption coefficient of the solvated electron band. These experimental results may be summarized as follows. (a) Bleaching and recovery of the solvated electron band in ammonia occurs within the pumping pulse width of 8 psec, and the symmetrical shape of the AD plots is undistorted from that of the pumping pulse. This indicates that bleaching and recovery proceed at least one order of magnitude more rapidly than the experimental time scale, i.e., 5 8 X sec. (b) When pumped by a 30 f 10 mJ 1.06-F pulse, the solvated electron band (monitored with 6.7-psec echelon pulses at 1100 nm) is bleached with an optical density change AD = 0.3 f 0.05 for a solution having DO = 1.3 a t The Journal of Physical Chemistry, Vol. 79, No. 26, 1975
2854
D. Huppert, P. M. Rentzepis, and W. S. Struve
O.'
I
0 -
I
t
-l
-o.2 -01
-
-02
-
-03
-
n 0
Q
I
-0.81
-18 -0 4 -
-12
-6
6
0
psec
6
12
18
24
30
36
t tpsec)
Figure 8. Time-resolved bleaching characteristics of the M- band of methylamine sodium solution at 630 nm. The interrogating light polarization is parallel to that of the bleaching beam.
-05
-
O6
TABLE I: Optical Density Changes (AD) in NH,-Na t, ND,-Na Solution Excited by a 1060-nm Pulse and 0
-1 5
T
and
Observed within the Time of the Pulse Width
15
(psec)
Figure 6. Time-resolved bleaching and recovery of the solvated electron absorption band of sodium deuterated ammonia solution at 1100 nm. In this case, the kinetics were monitored with 6.7 psec echelon/segments. All other experimental parameters same as Figure 3. 01
A, nm
NH ,
1100 1080 1040 1020 1000
-0.28 -0.18 -0.12
-0.10
960
-0.09
940
-0.08
ND-0.44 -0.34 -0.21
-0.17
0
-0 1
g
!
-02
0
-0 3
-04
-05
9000
10,000
11,000
Ati) Figure 7. Optical density changes (AD)of the ammonia-Na absorption band as a function of wavelength (A). The excitation was performed by a single 1060-nm pulse generated by a mode locked Nd3+ glass laser.
1100 nm, so that AD/D, = 0.23 f 0.05. If the bound electron's excited states has zero extinction coefficient at 1100 nm, ADID0 = x / ( l x ) , where x = 7tlI. Here E is the molar absorption coefficient at 1100 nm, I the 2-mm path length, and I = 4 X loz8 photdn/cm2 the photon flux. From this, the relaxation time 7 of the excited bound electron is estisec in NH3 at -7OOC. (c) The solmated to be -2 X vated electron. absorption band is homogeneously broadened between 850 and 1100 nm. The data plotted in Figure 7 are summarized in Table I.
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The Journal of Physical Chemistry, Vol. 79, No. 26, 1975
Methylamine. Bleaching of the Na- absorption band in Na-methylamine solution following 530-nm, 10-mJ excitation was monitored between 590 and 650 nm. Bleaching was proved for both interrogating pulse polarizations (orthogonal and parallel) relative to the excitation pulse polarization, in order to avoid errors in interpretation arising from dichroic effects. Figure 8 presents typical data for bleaching of the Na- band; while only data for parallel interrogation is shown, both polarizations yielded optical density changes of -1.2. The Na- band is clearly bleached immediately within experimental time resolution, and no significant recovery of the absorption occurs within 200 psec after excitation. Wavelength dependence studies of bleaching in the Na- band between 600 and 660 nm lead us to conclude that the Na- band is homogeneously broadened, in full accord with earlier nanosecond laser photolysis data.27 Formation of the solvated electron band was monitored between 770 and 1120 nm a t temperatures between -40 and -9OOC. This process, as is evidenced by the data in Figure 9, is completed within the 8 psec limit of the picosecond pulse width. Moreover, the solvated electron band does not decay within 200 psec; the normalized optical density changes plotted in Figure 10 vs. wavelength between 770 and 1120 nm, correspond quite wel1,to both the solvated electron absorption spectrum in methylamine and to the absorption band formed in the flash photolysis and nanosecond laser studies. Data for time-resolved bleaching and recovery of the solvated electron band by a 1.06-~single pulse, following the 530-nm photoionizing pulse by -80 psec, are typified by Figure 11 for Na-methylamine at -8OOC. Conclusions a-c
Picosecond Dynamics of Localized Electrons
2855
10 -
NO/Me-Amine 08
0.6 3
0
04
-
0
a
iooonm
-
0.2-
c
0.0
-
-80%
20 psec
I
530 nm
A 0
I ,
20
40
60
80
100
,
I
120
t lpsecl
Figure 9. Picosecond formation kinetics of the M- in methylamine sodium solution at I000 nm. The formathn of the M- band is immediately after excitation by the 530-nm pulse: solution temperature -8O’C
740
900
8CO
140
110c
(OW
~.nm
Figure 10. The optical density changes (AD) of the wavelength (A. nm) in methylamine sodium solution.
M- band vs.
20 pseclechelon.
of the preceding summarized data for the Na-NHZ resulta apply here as well, and we estimate, as in the Na-NH3 case, that regeneration of the solvated electron’s ground state occurs within -2 X sec. The degree of hleaching of the 1.3-p band hy a 1.06-p pulse, monitored between 850 and 1100 nm, follows the absorption cross section of the solvated electron in methylamine. This suggests, in analogy with the wavelength dependence studies of hleaching in Na”2, that the solvated electron hand is homogeneously hroadened in methylamine.
+ a
Discussion
o
20
40
80
80
m
120
no
im
t(PseC1
Any speculations concerning the relaxation mechanisms for excited trapped electrons io liquid ammonia and methylamine are necessarily entwined with assumptions about the levels populated by the 1.06-p pumping pulse. Though calculated configuration diagrams depicting the solvated electron’s 1s and 2p energies as a function of a symmetric cavity radius R have been offered as evidence that thermal fluctuations in R can lead to 1s 2p transition half-bandwidths commensurate with that ohserved,2 the possibility that the solvated electron absorption hand is inhomogeneously hroadened in liquid ammonia and methylamine is excluded hy our present data. Possible refinementa of the theory of line broadening in the 1s 2p and 1s np9 transitions include the dependence of the transition moment on the nuclear coordinates together with the incorporation of nontotally symmetric cavity distortions about the electron trapping center. Secondly, while the suhpicosecond relaxation time of -2 X 10-13 sec provides us with a first direct clue concerning these systems’ relaxation dynamics, we still cannot ascertain whether we are ohserving a hound-hound 1s 2p excitation followed by “predissociation-like” relaxation to the ground state, or whether a process involving creation of a localizing quasifree electron is involved. The trapping of a quasifree electron in polar solvents proceeds via initial trapping due to relaxation of long-range polar modes, followed by local relaxation of the first coordination layer which proceeds on the time scale of dielectric relaxation.18 The overall localization time in HzO at 300 K has been determined as -4 psec using picosecond spectroscopy,’8 while pulse radiolysis s t u d i e ~ ~led ~ . to * ~lifetimes of 2.2 psec in CH30H a t 300 K, 10 psec in C~HSOH a t 300 K, and -2 psec io H20 a t 236 K for the second stage of evolution of the solvated electron.
-
-
-
-
Figure 11. Bleaching kinetics of the electron band, M-, at 1000 nm. k is seen that t h e 1000-nm bleaching pulse is deiayed by EO psec with respect to the electron generating 530-nm pulse.
Since these are all approximately one order of magnitude longer than the relaxation time of the solvated electron pumped a t 1.06 p in liquid ammonia and methylamine, it is plausible that “intramolecular” type nonradiative relaxation to the ground state is occurring here. Independent studies of the relaxation of quasifree electrons in liquid ammonia are required to establish this, however. Our data indicate no change in the rate of formation of the solvated electron hand in methylamine following photoionization of Na- between the temperatures of -40 and -SO%, contrary to a priori expectations Eased on the hehavior of the solvated electron hand in water and the alcohols. Conceivable, the increase in dielectric relaxation time in methylamine within that viscosity range is insufficient to dilate the localization time by the one order’of magnitude necessary to he detected. Alternatively, hydrogen bonding may prove critical in determining the rate of the dielectric relaxation-limited second step of the localization process in the alcohols, and is much less prevalent in ammonia and the amines. This hypothesis is supported by the low temperature increase in localization time in a hydrogen-honded system, which is correlated with the known increase io hydrogen-bonding a t low temperatures. Acknowledgment. We gratefully acknowledge the many stimulating and helpful discussions with Professor J. Jortner and the valuable experimental efforts of Mr. J. Marshall. Th? J o w ~ l o f ~ ~ i c a l C h e m i r tVOl. r y .79. NO. 26. 1975
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D. Huppert, P. M. Rentzepis, and W. S. Struve
Discussion
References and Notes (1) W. L. Jolly, Ed., ”Metal-Ammonia Solutions”,Dowden, Hutchinson, and Ross, Inc., Stroudsburg, Pa., 1972. (2) (a)W. L. Jolly, C. J. Hallada, and M. Gold in “Metal-Ammonia Solutions”, Colloque Weyl I, G. Lepoutre and M. J. Sienko, Ed., W. A. Benjamin, New York, N.Y.. 1964. p 174; (b) H. Blades and J. W. Hodgins, Can. J. Chem., 33, 411 (1955). (3) (a) W. L. Jolly, Chem. Rev., 50, 351 (1952); (b) L. V. Couller, J. Phys. Chem., 57, 553 (1953); (c)L. V. Couller and L. Monchick, J. Am. Chem. SOC.,73,5867 (1951); and references therein. (4) (a) E. C. Ever and P. W. Frank, J. Chem. Phys. 26, 1517 (1957); (b) C. A. Kraus, J. Chem. Educ.. 30, 83 (1953). (5) D. E. O’Reilly, J. Chem. Phys., 35, 1856 (1962); (b) C. A. Hutchlnson and R. S. Pastor, ibid., 21, 1959 (1953); (c) S. Freed and N. Sugarman, ibid., 11, 354 (1943). (6) (a)H. M. McConnell and C. A. Holm, J. Chem. Phys. 26, 1517 (1957); (b) J. V. Acrivos and K. S.Pltzer, J. Phys. Chem., 66, 1693 (1962); (c)T. R. Hughes, J. Chem. Phys., 36, 202 (1962). (7) P. F. Rusch, W. H. Koehler. and J. J. Lagowski in “Metal-Ammonia Soiutions”, Colloque Weyl II. J. J. Lagowski and M. J. Sienko, Ed., Butterworths, London, 1970. (8) J. Jortner and N. R. Kestner in “Metal-Ammonia Solutions”, Colloque Weyl 11, J. J. Lagowski and M. J. Sienko, Ed., Butterworths, London, p 49, 1970. (9) I. Hurley, T. R. Tuttle, Jr., and S. Golden, “Metal-Ammonia Solutions”, Colloaue Weyl II. J. J. Lasowski and M. J. Sienko, Ed., Butterworths. London, p 449, 1970. (10) S. Matalon. S. Golden, and M. Ottolenghi, J. Phys. Chem.. 73, 3098 (1969). (11) See ref 9, p 476. (12) (a) R. A. Ogg, Phys. Rev., 69, 668 (1946); (b) R. A. Stairs, J. Chem. Phys., 27, 431 (1957); (c)J. Jortner, ibid., 30, 839 (1959). (13) J. Jortner, S. A. Rice, and E. G. Wilson in ”Metal-Ammonia Solutions”, Colloque Weyl I, G. Lepoutre and M. J. Sienko, Ed., W. A. Benjamln, New York, N.Y., p 222, 1964. (14) J. ’Jortner and N. R. Kestner in “Metal-Ammonia Solutions”, Colloque Weyl 11, J, J. Lagowski and M. J. Sienko, Ed., Butterworths, London, p 49, 1970. (15) N. R. Kestner and J. Jortner, J. Phys. Chem., 77, 1040 (1973). (16) We are not aware of any negative published report in the literature concerning this point, however, search for emission was performed by sev-
eral groups without any positive results.
(17) S. L. Hager and J. E. Willard, J. Chem. Phys., 61,3244 (1974). (18) P. M. Rentzepis, R. P. Jones, and J. Jortner, J. Chem. Phys., 59, 9 (1973). (19) L. Gilles, J. E. Aldrich, and J. W. Hunt, Nature (London),Phys. Sci.. 243, 70 (1973). (20) J. H. Baxendale and P. Wardman, J. Chem. SOC.,Faraday Trans. 1, 69, 584 (1973). (21) These pulse widths were ascertained in a separate three-photon fluorescence experiment; see P. M. Rentzepis, C. J. Mitschele, and A. C. Saxman, Appl. Phys. Lett., 17, 122 (1970). (22) T. L. Netzel, W. S. Struve, and P. M. Rentzepis, Annu. Rev. Phys. Chem., 24,473 (1973). (23) F. Shimlzu, IBM J. Res. Dev., 17, 286 (1973), and references cited
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J. JORTNER. Confronting the experimental data reported by the Bell group (relaxation time 7 2 x 10-13 sec) for excitation a t 1.06 p with the results of the Orsay group (7 I5 X 10-l’sec) for excitation a t 6100 A, it appears that the relaxation times are different. The former experiment corresponds to bound (2p) to bound (1s) relaxation while the latter data pertain to ionization followed by (“two step”) relaxation to the ground 1s state. Furthermore, the observation of homogeneous broadening (or bleaching) of the absorption bond in the range 8000-11000 8, provides support to the theoretical ideas that the 1s 2p absorption line shape is broadened by the conventional phonon coupling mechanism. What is still missing are optical bleaching experiments at 1.P1.5 fi. If this experiment yields identical relaxation times with those observed by excitation a t 1.06 p, we shall have strong support for the identification of the broad absorption band 2p bound-bound in that range with the phonon broadened Is transition.
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T. TUTTLE.(1) How does the value of the relaxation time distinguish between the bound state and the continuum? (2) Does exciting into the continuum give a different relaxation time? (3) What is the mechanism for this very rapid relaxation? J. JORTNER. The mechanism of electron localization in a polar liquid bears a close analogy to the well-known electron-hole recombination in semiconductors, as studied by Kubo, Toyozawa, and others. The basic physics involved is a nonradiative multiphonon relaxation of the electron from an initial quasifree state to the final bound state. The excess energy is converted into vibrational energy which will involve polar phonon modes of the solvent and possibly also intramolecular vibrational modes of the solvent molecules. P. DELAHAY.Question to Jortner: Why would you feel reassured by an experiment that shows that you have a bound state? The theoretical problem is a much broader one, anyhow. J. JORTNER. In response to the comment made by P. Delahay, let me emphasize that when we have complete relaxation data for excitation of electrons in ammonia a t 1.4-1.5 p (TI),a t 1.06 p ( ~ d and at 5300 8, (TS), and provided that 71 = 72 < 73, we shall have a strong basis for distinguishing between bound-bound and boundcontinuum excitation, as predicted by current theories.
ing of the echelon images.
J. L. DYE. The magnitude of the optical density changes at 600 and a t 1000 nm are of interest since the values shown seem to indi2e- + Na+ (response indicates different intensities). cate NaIt would be of interest of measure AD at both wavelengths with the same intensity pulse in order to determine the extinction coefficients.
ration.
D. HUPPERT. The ratio ADsoo/ADl~ 3 was measured in 1,3propylenediamine solution with 20-nsec time resolution. This work appeared in J. Phys. Chem., 74,3285 (1970).
therein.
(24) C. G. 0. Varma and P. M. Rentzepis, J. Chem. Phys., 58, 5237 (1973); M. Clerc, R. P. Jones, and P. M. Rentzepis, Chem. Phys. Lett., 26, 167 (1974). (25) A continuous wave Nd3+:YAGlaser was used for alignment and focus(26) M. A. Duguay and J. W. Hansen, AppI. Phys. Lett., 15, 192 (1969). (27) D. Huppert and K. H. Bar-Eli, J. Chem. Phys., 74, 3285 (1970). (28) K. J. Xaufmann. D. Huppert, and P. M. Rentzepls, manuscript in prepa-
The Journal of Physical Chemistry, Vo/. 79, No. 26, 1975
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