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A giant-dipole laser in a resonator-transient configuration pumped by a picosecond laser
Volume128, number5,6
CHEMICALPHYSICS LETTERS
8
August1986
A GIANT-DIPOLE LASER IN A RESONATOR-TRANSIENT CONFIGURATION PUMPED BY A PICOSECOND LASER Hisao UCHIKI and Takayoshl KOBAYASHI Department of Physics, Faculty of Science, University of Tokyo, Hongo 7-3-1, Bunkyo-ky
Tokyo 113, Japan
Received 4 April 1986; in final form 7 May 1986
The time-resolved laser spectrum of 4d~ethyl~~o4‘-~tro~~bene in benzene pumped by a 30 ps, 355 mn &&t pulse was measured. The laser had a short-cavity, resonator-transient configuration. No spectral shift was observed during laser oscillation. This is consistent with the result of the analysis of lasing by simplified rate equations.
1. Introduction Spectralshifts of dye-laser emission by c~nging solvents are especially large for dyes where dipole moments differ appreciably between the ground and the excited states [ 11. The dye molecules in a nonviscous solvent are excited in the non-equilibrium Franck-Condon states following light absorption, and they relax to the equilibrium states within 0.1 to 10 ps [2,3]. One of the typical molecules with these properties is 4dimethylamino4’nitrostilbene (DMANS). It has a dipole moment of 7.6 D in the ground state and 32 D in the lowest excited singlet state 141. The various solutions of DMANS were reported to oscillate by the use of a N2 laser as a pumping source [5,6]. The tuning range extended to 130 nm by changing solvents [5,6]. This is much larger than the tuning range (50-60 nm) of ordinary dye lasers such as a rhodamine 6G laser. Recently, we studied picosecond absorption and gain spectra of a benzene solution and of a polymeric film containing 4die~y~mino4’-nitrostilbene (DEANS), which has similar optical properties to DMANS [7 ] . We also studied the Iasing properties and the nanosecond gain spectrum of DEANS in solutions of different polarity [8]. In the case of pumping dye lasers by a nanosecond light pulse, the populations of the FranckCondon states are negligibly small, and the rate equations representing the dye laser oscillation in 0 ~9.2614~86~$03.50 0 Elsevier Science Publishers B.V. ~o~h-Ho~~d Physics Pub~~g Division)
a four-level system reduce to the two-level system of the ground and the excited equilibrium states. However, in the case of pumping a dye laser in a short-cavity con~guration by a picosecond light pulse or of strong excitation by an intense pumping pulse, the populations of the Franck-Condon states may not be neglected, It is expected that the populations of the Franck-Condon states cause a decrease in gain of the laser medium and make the duration of the laser pulse longer. The time-resolved laser spectrum in the picosecond regime will give information on the kinetics of the laser action of molecules with a large dipole moment in the excited state and on the effects of solvents on la&g. In the present paper we report lasing properties and picosecond time-resolved laser spectra of a DMANS (in benzene) laser in a short-cavity, resonator-transient configuration pumped by the third harmonic (355 nm) of a Nd:YAG laser with a fwhm of 30 ps. Three-dimensional intensity profiles of the laser as a function of wavelength and delay time after pumping were measured at various pumping intensities by the use of a streak camera in the twodimensional mode.
2. Experimental The configuration of the laser used in the present study is shown in fig. 1, which is similar to that re559
Volume 128, number 5,6
CHEMICAL PHYSICS LETTERS
QUARTZ CELLklmm)
8 August 1986
length- and time-resolved intensity from the DMANS laser is shown in figs. 2 and 3 in the forms of contour
DMdNSIBENZENE
Fig. 1. Configuration of the DMANS laser.
ported in the literature [9]. A benzene solution of DMANS in a fused quartz cell was longitudinally pumped through anf = 25 cm lens by the third harmonic (355 nm) of a Nd:YAG laser (Quantel, YG472) with 30 ps fwhm and 200 CUJenergy. The concentration of DMANS in benzene was 1 X low2 mol/dms . The internal cell length is 1 mm and the thickness of the cell walls is 1 mm. An aluminum mirror was attached to the rear glass plate of the cell. The angle between the pump and laser beams is 4”. The reflected pumping light was eliminated by a prism and a set of color filters. The laser was focused on the entrance slit of an f = 25 cm polychromator (Unisoku) with a multichannel photodiode array (MCPD) on the exit focal plane for the measurement of a time-integrated laser spectrum. The spectral data detected by the MCPD were digitized by a 12.bit AD converter and transferred to a microcomputer. For the measurement of a time-resolved laser spectrum, the laser beam was focused on the entrance slit of anf= 10 cm polychromator (Ritsu, MClO). The output light from the polychromator was focused on a 100 p slit of a streak camera (Hamamatsu, C1370) operating in a twodimensional (one dimension for time sweeping and the other for wavelength) mode. The video signal of the image received by the streak camera was digitized and transferred to a microcomputer . The temporal and spectral resolutions of the combined system of the polychromator and the streak camera were 20 ps and 1.5 mn, respectively.
62c
100
0 TIME(ps)
Fig. 3. Contour map of the threedimenaionally displayed wavelength-resolved time dependence of the DMANS,laser intensity. The pumping energy is 130 CJ. The relative intensities normalized to 10 at the top are given by numerical figures.
65
I
f
9 w d p
63
3: f
3. Results and discussion 3.1. Time-resolved laser spectrum of DMAiVS in benzene
The threshold pumping energy of the DMANS laser was about 40 @. The laser pulse energy was 0.1 fl when the pumping energy was 170 ErJ.The wave-
62c
0 TIME (ps)
100
Fig. 2. Contour map of the three-dimensionally displayed wavelength-resolved time dependence of the DMANS laser intensity near the oscillation threshold. The pumping energy is 40 rJ. The relative intensities normalized to 10 at the top are given by numerical fiiures.
CHEMICAL PHYSICS LETTERS
Volume 128, number 5,6
petted to be of the order of a few picoseconds to ten
maps of the threedimensional displays. The pumping energies are 40 (threshold pumping energy) and 130 /J.Ifor figs. 2 and 3, respectively. The relative intensities normalized to 10 at the top are given by successively decreasing numerical figures. The wavelength of the peak remains constant at 632 nm in fg 3. As the pumping energy increases, the laser oscillation starts at shorter delay times after pumping and the pulse width (fwhm) decreases from 130 ps for 50 @ pumping to 85 ps for 130 @. However, the wavelength shift of the peak during laser emission was not observed for all the pumping energies up to about 190 /J.I.In fig. 4, the wavelength at the maxi; mum intensity in the time-integrated laser spectrum is shown for various pumping energies. The spectrum does not change with increase in pumping energy. The laser can be modeled as a four-level system consisting of S,,(Ql), S, (Ql), S, (Q2), and So(Q2) where Ql and Q2 are the equilibrium configurations in the ground state, So, and the lowest excited state, S, , respectively. Immediately after excitation, the molecules are in a non-equilibrium (Franck-Condon) state, S, (Ql) (n > l), and they relax to the equilibrium (excited) state, S, (Q2), within picoseconds. The relaxation time from S, (Q2) to Su (Ql) is ex-
picoseconds .
Here we discuss the conditions for the spectral shift to be observed during laser emission. Following are three cases, (i) to (iii), of the pumping light intensity and duration. (i) The pumping light has a shorter width (rp) than the reorientation relaxation time (t,). (ii) The pumping light pulse, with a width longer than t,, has a power high enough for the bleaching of the population of the ground state to take place within times shorter than t,. In either of these cases, the following two conditions must be satisfied. (A) The build-up time of the laser oscillation must be shorter than t, and (B) the induced emission rate during laser oscihation must be slightly higher than l/t,. When (iii) the pumping light pulse, with a width longer than tr , has a power which is not high enough to satisfy case (ii), condition (B) must be satisfied. Only two cases, (ii) and (iii), will be discussed below since the pulse width (tP = 30 ps) is longer than I, (< 8 ps [lo]). The build-up time can be estimated by the following simplified rate equations. The rate equation for the population, n, , of the upper laser level is dn,/dt = uaIp(t)no/hvp - n,u,cq/n
-I E P 5 w 6302 3
dqldt = nuoecq/n - q/tc + gn,ltr ,
'
'
- nu/tf ,
(1)
where Ip (t) is the time dependence of the pumping intensity, ua is the absorption cross section of the DMANS molecule at the pumping photon energy (hvp), no is the population of the ground state (S,(Ql)), ue is the effective stimulated emission cross section at the lasing wavelength given by ulo - a* where ulo is the cross section of stimulated emission SI + So and u* the cross section of absorption S, f S1 , c is the velocity of light in vacuum, q is the photon number density inside the cavity, rf is the fluorescence lifetime, and n is the refractive index of the solution. The equation of motion for the photon number density is
6&O-
620;
8 August 1986
'
'
' 100
'
'
'
'
'
200
PUMP’ENERGY (pJ1 Fig. 4. Dependence of the wavelength of maximum intensity in the time-integrated DMANS laser spectrum on the pumping energy.
'
(2)
where t, is the cavity photon lifetime, and g is the fraction of the spontaneous fluorescence fed into the laser mode. Here we estimate the build-up time of the laser oscillation, c,, , after pumping by a short light pulse of delta-functional form, by which the molecules are fully pumped as n, (t = 0) = N (total number density of molecules). We define the buildup time as n,(t = tb) = O.gn,(O). For t < lb, n, is 561
CHEMICAL PHYSICS LETTERS
Volume 128, number 5,6
considered to be nearly constant in the time region of interest, and eqs. (1) and (2) can easily be integrated, the results being 4(r) =gn/tfoeC(exp[(n,oeCln
n,(t) = n, (0)exp -
u,c/n
t
-
l/tJfl - 119 (34
sq(t’) dt’ - t/q
, (3b)
0
The build-up time is obtained by the following equation :
ntf(n,u,cln - l/t,>
8 August 1986
of the former to the latter changes shot by shot, but is less than about 10% on the average. Since there is no wavelength selector in the cavity, it is the vibrational structure of a DMANS molecule. In conclusion, we have studied lasing properties of a DMANS/benzene laser in a short cavity, resonatortransient configuration pumped by a 30 ps Nd:YAG laser at 355 nm. From the measurement of the timeresolved emission spectra it is found that there is no spectral shift during laser oscillation. The effect of the reorientation relaxation of solvent molecules was not observed. This is because of both the fast reorientation compared with the onset of laser oscillation and the small induced emission rate at the actual pumping level in the experiment.
exp[(n,u,C/n - l/t,) tb 1 - 1 n,u,c/n
- l/t, Adcnowledgement
+ t&f
= -h(o.9).
(4)
In the typical case, the parameters are n = 1.5 ,N = 6 X lOI cmm3 (1 X low2 mol/dm3), ue = 1 X lo-l6 cm2,tf=3,3ns [ll],g= f(r/L)2,r=0.1mm,L=1 mm,andt,=7ps.Inthiscase,n,ueis6X 102cm-l and the build-up time calculated from eq. (4) is 12 ps. For the pumping intensities with n, ue = 1 X 102, 6 X 10 and 3 X 10 cm-l, the build-up times are 7,11 and 23 ps, respectively. Since delta-functional pumping is assumed, the estimated buildup times offer lower limits of the pulse width obtained by actual excitation. In the case of pumping with 30 ps pulses with the same pulse energy as the delta-functional pulse, the build-up times are longer than that estimated from eq. (4). The observed induced emission rate was about 6 X lo8 s-l for the 85 ps laser output at 630 nm with a pulse energy of 0.1 fl. Therefore the conditions necessary for the spectral shift to be observed are not satisfied in the present case, and this is consistent with experimental observations. The other characteristic of the DMANS laser is that there is a substructure irr the emission spectrum for pumping energies smaller than about 100 fl. There are two peaks at 620 and 630 nm, which have the same growth time constant. The intensity ratio
562
This research was partly supported by a Grant-inAid for Special Distinguished Research (No. 56222005) from the Ministry of Education, Science and Culture, the Toray Science Foundation, the Kajima Foundation , and the Kurata Foundation.
References [ 1] F.P. Schafer, W. Schmidt and J. Volze, Appl. Phys. Letters 9 (1,966) 306.
[ 21 T. Kobayashi, S. Nagakura and M. Szwarc, Chem. Phys. 39 (1979) 105. [3] T. Kobayashi,Chem. Phys. Letters 85 (1982) 170. [4] E. Lippert, Z. Elektrochem. 61(1957) 962. [5] R. KGnig, St. Mory, M. Scholtz and D. Leupold, Exp. Tech. Phys. 25 (1977) 195. [6] St. Mory and H. Becker-Ross, Exp. Tech. Phys. 27 (1979) 359. [7] T. Kobayashi, H. Ohtani and K. Kurokawa, Chem. Phys. Letters 121(1985) 356. [ 81 T. Kobayashi, M. Terauchi and H. Uchiki, Chem. Phys. Letters 126 (1986) 143. [9] P.H. Chiu, S. Hsu, S.J.C. Box and H.-S. Kwok, IEEE J. Quantum Electron. QE-20 (1984) 652. [lo] T. Kobayashi, E.O. Degenkolb and P.M. Rentzepis, J. Appl. Phys. 50 (1979) 3118. [ 1 l] P.P. Shorygin and T.M. Ivanova, Soviet Phys. Dokl. 3 (1958) 764.
CHEMICALPHYSICS LETTERS
8
August1986
A GIANT-DIPOLE LASER IN A RESONATOR-TRANSIENT CONFIGURATION PUMPED BY A PICOSECOND LASER Hisao UCHIKI and Takayoshl KOBAYASHI Department of Physics, Faculty of Science, University of Tokyo, Hongo 7-3-1, Bunkyo-ky
Tokyo 113, Japan
Received 4 April 1986; in final form 7 May 1986
The time-resolved laser spectrum of 4d~ethyl~~o4‘-~tro~~bene in benzene pumped by a 30 ps, 355 mn &&t pulse was measured. The laser had a short-cavity, resonator-transient configuration. No spectral shift was observed during laser oscillation. This is consistent with the result of the analysis of lasing by simplified rate equations.
1. Introduction Spectralshifts of dye-laser emission by c~nging solvents are especially large for dyes where dipole moments differ appreciably between the ground and the excited states [ 11. The dye molecules in a nonviscous solvent are excited in the non-equilibrium Franck-Condon states following light absorption, and they relax to the equilibrium states within 0.1 to 10 ps [2,3]. One of the typical molecules with these properties is 4dimethylamino4’nitrostilbene (DMANS). It has a dipole moment of 7.6 D in the ground state and 32 D in the lowest excited singlet state 141. The various solutions of DMANS were reported to oscillate by the use of a N2 laser as a pumping source [5,6]. The tuning range extended to 130 nm by changing solvents [5,6]. This is much larger than the tuning range (50-60 nm) of ordinary dye lasers such as a rhodamine 6G laser. Recently, we studied picosecond absorption and gain spectra of a benzene solution and of a polymeric film containing 4die~y~mino4’-nitrostilbene (DEANS), which has similar optical properties to DMANS [7 ] . We also studied the Iasing properties and the nanosecond gain spectrum of DEANS in solutions of different polarity [8]. In the case of pumping dye lasers by a nanosecond light pulse, the populations of the FranckCondon states are negligibly small, and the rate equations representing the dye laser oscillation in 0 ~9.2614~86~$03.50 0 Elsevier Science Publishers B.V. ~o~h-Ho~~d Physics Pub~~g Division)
a four-level system reduce to the two-level system of the ground and the excited equilibrium states. However, in the case of pumping a dye laser in a short-cavity con~guration by a picosecond light pulse or of strong excitation by an intense pumping pulse, the populations of the Franck-Condon states may not be neglected, It is expected that the populations of the Franck-Condon states cause a decrease in gain of the laser medium and make the duration of the laser pulse longer. The time-resolved laser spectrum in the picosecond regime will give information on the kinetics of the laser action of molecules with a large dipole moment in the excited state and on the effects of solvents on la&g. In the present paper we report lasing properties and picosecond time-resolved laser spectra of a DMANS (in benzene) laser in a short-cavity, resonator-transient configuration pumped by the third harmonic (355 nm) of a Nd:YAG laser with a fwhm of 30 ps. Three-dimensional intensity profiles of the laser as a function of wavelength and delay time after pumping were measured at various pumping intensities by the use of a streak camera in the twodimensional mode.
2. Experimental The configuration of the laser used in the present study is shown in fig. 1, which is similar to that re559
Volume 128, number 5,6
CHEMICAL PHYSICS LETTERS
QUARTZ CELLklmm)
8 August 1986
length- and time-resolved intensity from the DMANS laser is shown in figs. 2 and 3 in the forms of contour
DMdNSIBENZENE
Fig. 1. Configuration of the DMANS laser.
ported in the literature [9]. A benzene solution of DMANS in a fused quartz cell was longitudinally pumped through anf = 25 cm lens by the third harmonic (355 nm) of a Nd:YAG laser (Quantel, YG472) with 30 ps fwhm and 200 CUJenergy. The concentration of DMANS in benzene was 1 X low2 mol/dms . The internal cell length is 1 mm and the thickness of the cell walls is 1 mm. An aluminum mirror was attached to the rear glass plate of the cell. The angle between the pump and laser beams is 4”. The reflected pumping light was eliminated by a prism and a set of color filters. The laser was focused on the entrance slit of an f = 25 cm polychromator (Unisoku) with a multichannel photodiode array (MCPD) on the exit focal plane for the measurement of a time-integrated laser spectrum. The spectral data detected by the MCPD were digitized by a 12.bit AD converter and transferred to a microcomputer. For the measurement of a time-resolved laser spectrum, the laser beam was focused on the entrance slit of anf= 10 cm polychromator (Ritsu, MClO). The output light from the polychromator was focused on a 100 p slit of a streak camera (Hamamatsu, C1370) operating in a twodimensional (one dimension for time sweeping and the other for wavelength) mode. The video signal of the image received by the streak camera was digitized and transferred to a microcomputer . The temporal and spectral resolutions of the combined system of the polychromator and the streak camera were 20 ps and 1.5 mn, respectively.
62c
100
0 TIME(ps)
Fig. 3. Contour map of the threedimenaionally displayed wavelength-resolved time dependence of the DMANS,laser intensity. The pumping energy is 130 CJ. The relative intensities normalized to 10 at the top are given by numerical figures.
65
I
f
9 w d p
63
3: f
3. Results and discussion 3.1. Time-resolved laser spectrum of DMAiVS in benzene
The threshold pumping energy of the DMANS laser was about 40 @. The laser pulse energy was 0.1 fl when the pumping energy was 170 ErJ.The wave-
62c
0 TIME (ps)
100
Fig. 2. Contour map of the three-dimensionally displayed wavelength-resolved time dependence of the DMANS laser intensity near the oscillation threshold. The pumping energy is 40 rJ. The relative intensities normalized to 10 at the top are given by numerical fiiures.
CHEMICAL PHYSICS LETTERS
Volume 128, number 5,6
petted to be of the order of a few picoseconds to ten
maps of the threedimensional displays. The pumping energies are 40 (threshold pumping energy) and 130 /J.Ifor figs. 2 and 3, respectively. The relative intensities normalized to 10 at the top are given by successively decreasing numerical figures. The wavelength of the peak remains constant at 632 nm in fg 3. As the pumping energy increases, the laser oscillation starts at shorter delay times after pumping and the pulse width (fwhm) decreases from 130 ps for 50 @ pumping to 85 ps for 130 @. However, the wavelength shift of the peak during laser emission was not observed for all the pumping energies up to about 190 /J.I.In fig. 4, the wavelength at the maxi; mum intensity in the time-integrated laser spectrum is shown for various pumping energies. The spectrum does not change with increase in pumping energy. The laser can be modeled as a four-level system consisting of S,,(Ql), S, (Ql), S, (Q2), and So(Q2) where Ql and Q2 are the equilibrium configurations in the ground state, So, and the lowest excited state, S, , respectively. Immediately after excitation, the molecules are in a non-equilibrium (Franck-Condon) state, S, (Ql) (n > l), and they relax to the equilibrium (excited) state, S, (Q2), within picoseconds. The relaxation time from S, (Q2) to Su (Ql) is ex-
picoseconds .
Here we discuss the conditions for the spectral shift to be observed during laser emission. Following are three cases, (i) to (iii), of the pumping light intensity and duration. (i) The pumping light has a shorter width (rp) than the reorientation relaxation time (t,). (ii) The pumping light pulse, with a width longer than t,, has a power high enough for the bleaching of the population of the ground state to take place within times shorter than t,. In either of these cases, the following two conditions must be satisfied. (A) The build-up time of the laser oscillation must be shorter than t, and (B) the induced emission rate during laser oscihation must be slightly higher than l/t,. When (iii) the pumping light pulse, with a width longer than tr , has a power which is not high enough to satisfy case (ii), condition (B) must be satisfied. Only two cases, (ii) and (iii), will be discussed below since the pulse width (tP = 30 ps) is longer than I, (< 8 ps [lo]). The build-up time can be estimated by the following simplified rate equations. The rate equation for the population, n, , of the upper laser level is dn,/dt = uaIp(t)no/hvp - n,u,cq/n
-I E P 5 w 6302 3
dqldt = nuoecq/n - q/tc + gn,ltr ,
'
'
- nu/tf ,
(1)
where Ip (t) is the time dependence of the pumping intensity, ua is the absorption cross section of the DMANS molecule at the pumping photon energy (hvp), no is the population of the ground state (S,(Ql)), ue is the effective stimulated emission cross section at the lasing wavelength given by ulo - a* where ulo is the cross section of stimulated emission SI + So and u* the cross section of absorption S, f S1 , c is the velocity of light in vacuum, q is the photon number density inside the cavity, rf is the fluorescence lifetime, and n is the refractive index of the solution. The equation of motion for the photon number density is
6&O-
620;
8 August 1986
'
'
' 100
'
'
'
'
'
200
PUMP’ENERGY (pJ1 Fig. 4. Dependence of the wavelength of maximum intensity in the time-integrated DMANS laser spectrum on the pumping energy.
'
(2)
where t, is the cavity photon lifetime, and g is the fraction of the spontaneous fluorescence fed into the laser mode. Here we estimate the build-up time of the laser oscillation, c,, , after pumping by a short light pulse of delta-functional form, by which the molecules are fully pumped as n, (t = 0) = N (total number density of molecules). We define the buildup time as n,(t = tb) = O.gn,(O). For t < lb, n, is 561
CHEMICAL PHYSICS LETTERS
Volume 128, number 5,6
considered to be nearly constant in the time region of interest, and eqs. (1) and (2) can easily be integrated, the results being 4(r) =gn/tfoeC(exp[(n,oeCln
n,(t) = n, (0)exp -
u,c/n
t
-
l/tJfl - 119 (34
sq(t’) dt’ - t/q
, (3b)
0
The build-up time is obtained by the following equation :
ntf(n,u,cln - l/t,>
8 August 1986
of the former to the latter changes shot by shot, but is less than about 10% on the average. Since there is no wavelength selector in the cavity, it is the vibrational structure of a DMANS molecule. In conclusion, we have studied lasing properties of a DMANS/benzene laser in a short cavity, resonatortransient configuration pumped by a 30 ps Nd:YAG laser at 355 nm. From the measurement of the timeresolved emission spectra it is found that there is no spectral shift during laser oscillation. The effect of the reorientation relaxation of solvent molecules was not observed. This is because of both the fast reorientation compared with the onset of laser oscillation and the small induced emission rate at the actual pumping level in the experiment.
exp[(n,u,C/n - l/t,) tb 1 - 1 n,u,c/n
- l/t, Adcnowledgement
+ t&f
= -h(o.9).
(4)
In the typical case, the parameters are n = 1.5 ,N = 6 X lOI cmm3 (1 X low2 mol/dm3), ue = 1 X lo-l6 cm2,tf=3,3ns [ll],g= f(r/L)2,r=0.1mm,L=1 mm,andt,=7ps.Inthiscase,n,ueis6X 102cm-l and the build-up time calculated from eq. (4) is 12 ps. For the pumping intensities with n, ue = 1 X 102, 6 X 10 and 3 X 10 cm-l, the build-up times are 7,11 and 23 ps, respectively. Since delta-functional pumping is assumed, the estimated buildup times offer lower limits of the pulse width obtained by actual excitation. In the case of pumping with 30 ps pulses with the same pulse energy as the delta-functional pulse, the build-up times are longer than that estimated from eq. (4). The observed induced emission rate was about 6 X lo8 s-l for the 85 ps laser output at 630 nm with a pulse energy of 0.1 fl. Therefore the conditions necessary for the spectral shift to be observed are not satisfied in the present case, and this is consistent with experimental observations. The other characteristic of the DMANS laser is that there is a substructure irr the emission spectrum for pumping energies smaller than about 100 fl. There are two peaks at 620 and 630 nm, which have the same growth time constant. The intensity ratio
562
This research was partly supported by a Grant-inAid for Special Distinguished Research (No. 56222005) from the Ministry of Education, Science and Culture, the Toray Science Foundation, the Kajima Foundation , and the Kurata Foundation.
References [ 1] F.P. Schafer, W. Schmidt and J. Volze, Appl. Phys. Letters 9 (1,966) 306.
[ 21 T. Kobayashi, S. Nagakura and M. Szwarc, Chem. Phys. 39 (1979) 105. [3] T. Kobayashi,Chem. Phys. Letters 85 (1982) 170. [4] E. Lippert, Z. Elektrochem. 61(1957) 962. [5] R. KGnig, St. Mory, M. Scholtz and D. Leupold, Exp. Tech. Phys. 25 (1977) 195. [6] St. Mory and H. Becker-Ross, Exp. Tech. Phys. 27 (1979) 359. [7] T. Kobayashi, H. Ohtani and K. Kurokawa, Chem. Phys. Letters 121(1985) 356. [ 81 T. Kobayashi, M. Terauchi and H. Uchiki, Chem. Phys. Letters 126 (1986) 143. [9] P.H. Chiu, S. Hsu, S.J.C. Box and H.-S. Kwok, IEEE J. Quantum Electron. QE-20 (1984) 652. [lo] T. Kobayashi, E.O. Degenkolb and P.M. Rentzepis, J. Appl. Phys. 50 (1979) 3118. [ 1 l] P.P. Shorygin and T.M. Ivanova, Soviet Phys. Dokl. 3 (1958) 764.