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Quantum efficiency and thermal stability of organic dye vapors
QUANTUM EFFICIENCY AND THERMAL STABILITY
Takeki SAKURAI, Department
April 1978
OPTICS COMMUNICATIONS
Volume 25, number 1
OF ORGANIC DYE VAPORS
Atsushi OGISHIMA and Masao SUGAWARA
of Electronic
Engineering,
Faculty of Engineering,
Yamanashi University, Kofu, Japan
Received 28 November 1977 Organic dyes, POPOP, perylene and pyren, in the vapor phase are excited by a Nz laser. The quantum efficiency is measured as a function of temperature up to 45O’C. Optical properties such as the radiative transition probability are also estimated from the experimental results. It is found that the decrease in the vapor density by the thermal dissociation can be neglected during several tens of hours.
The vapor phase dye laser has been reported with various materials [I -41. Temperature is the main parameter to determine the power of this type of laser. Though the vapor pressure generally increases almost exponentially with the temperature, the power of the laser has a peak at some temperature. The dissociation of molecules and radiationless transition may be important at high temperature. These processes are deeply related with the quantum efficiency of fluorescence and the stability of the dye vapor. We report here detailed measurements of optical properties such as the quantum efficiency and the stability of POPOP, perylene and pyren in the vapor phase. POPOP is the most popular substance as a vaporphase dye laser and perylene and pyren, with which a laser oscillation is not obtained in the vapor phase, are examined in comparison with POPOP. The quantum efficiency of POPOP has been measured at temperatures below 330°C and reported to decrease with temperatures [5]. It is considered desirable to examine the change in the efficiency up to 450°C as described in this paper, since the peak of the laser power is obtained at about 380°C. In the case of perylene only a few data of optical properties in the vapor phase have been reported [6-81 and data are not enough to analyze an excitation or emission mechanism. Optical properties of pyren in the vapor phase are not well known. Crystals of dye molecules were inserted in a glass cilindrical cell with diameter of 2.0 cm and length of 2.5 cm, the cell was sealed off under vacuum and set in a heater box. Dye vapors in the cell were excited by
a N2 laser with wavelength of 337.1 nm and duration of 10 ns. A fluorescence from the dye near the entrance window for the exciting laser was detected by a photomultiplier after passing a monochromator. At the same time the absorption coefficient of the dye at 337.1 nm was measured. The amount of POPOP, for example, in the cell was so small that the density of the vapor was equal to 6.0 X 1015 cme3 when all of POPOP were evaporated. The amount was determined to keep the absorption coefficient almost constant at temperatures above 300°C. This makes it simple to measure the quantum efficiency up to 450°C. The cross section of absorption at 337.1 nm is also calculated from the constant absorption coefficient and the vapor density. In the experiment the intensity of the N2 laser was kept to be small enough not to saturate the ground state of the dye. The absorption coefficient and the fluorescence intensity of POPOP vapor measured at the peak of the fluorescence bands, 38.5 nm, and the peak of the duration are shown in fig. I. The lines shape was independent of the temperature. It is clear from fig. 1 that the quantum efficiency decreases with the temperature. Under the condition that the ground state of the dye is not saturated the rate equation describing the density of the first excited singlet state is easily solved, if the intensity profile of the pumping N2 laser, P, is given as P = PO sinot
O
P=O
n/o G t.
Ions, (1)
75
Volume
25. number
1
OPTICS COMMUNICATIONS
April 1978
iF~:--:::_ 0
+.
04:
250
300
350
400
TEMPERATURE
Fig. 1. Peak fluorescence intensity1 and absorption coefficient Q at 337.1 nm of POPOP vapor as a F unction of temperature. Vapor density above 350°C is 6.0 X 10” cme3.
When the peak density solved is denoted by F(y),,;n, the fluorescence intensity at the peak of duration, Ip, is shown by
$ = ~~ -we)
eY),,lX,,
(2)
where n is the total density of the dye, CYis the absorption coefficient at 337.1 nm,A is the radiative transition probability which is equal to the reciprocal of the natural life time, A, is the wavelength of the fluorescence, L(X,) is the normalized line shape, y is the total decay rate of the first excited state which is equal to the reciprocal of the fluorescence life time and k is a proportionality constant. The fluorescence intensity, the absorption coefficient and the line shape are measured from the experiment. Therefore, from the ratio of the fluorescence intensity of the dye vapor to POPOP in a liquid solution one unknown parameter A or y in the vapor phase can be estimated, because values of ys and As of POPOP in the liquid solution are known to be 6.7 X 1O8 s-l and 5.7 X I O8 s-l, respectively [9] . The quantum efficiency @P is given by A/y. The probability A of POPOP in the vapor phase is assumed to be equal to A, from the fact that the emission and the absorption curves show nearly perfect mirror symmetry and the absorption cross section integrated over the line shape is nearly equal to that in the solution. The decay rate and the quantum efficiency of POPOP estimated are shown in fig. 2 as a function of temperature. Values at temperatures below 300°C seem to agree with those in the solution which are denoted on the axis of ordinate in fig. 2. This is consistent with the results in ref. [S] . The efficiency at 450°C is almost one fourth of that in the solution. 76
Fig. 2. Total decay rate 7 and quantum cence @F of POPOP vapor as a function
450
(‘C)
efficiency of fluoresof temperature.
Fluorescence intensities of perylene and pyren at a constant vapor density were measured to be independent of temperatures within the experimental error. This indicates that the efficiency #F is constant and independent of temperature. It was verified from the line shape and the decay curve that observed emissions do not originate from a excimer [IO] and are not a delayed fluorescence [I I ] . The decay rate y of pyren was directly measured from the decay time of the fluorescence, which was much longer than the duration of the N, laser and the time resolution of the detector. It was confirmed from this direct measurement that y was independent of temperature. The decay rate of perylene in the vapor phase has been measured to be 2.5 X IO* s-l [8] . This value almost agreed with our result of the direct measurement under the consideration of the duration and the time resolution. Values of fundamental parameters obtained are listed in table 1. The radiative probability of perylene or pyren vapor roughly agrees with that in the solution [ 12- 151. The value A of perylene also agrees with the result in the vapor phase in ref. [8] but is four times as large as that in ref. [7]. The result in ref. [7] may be too small. The quantum efficiency of perylene is in fair agreement with the previous results in the vapor phase, 0.32 [6], 0.30 [7] and0.39 [8].H owever efficiencies of perylene and pyren in table 1 are considerably smaller than those in the solution [12-IS]. The thermal stability of the dye vapor is estimated from the time dependence of the absorption coefficient. When the cell temperature is so high that all of dye is evaporated, the absorption coefficient directly shows a change in the density of the ground state. The absorption coefficient at a constant temperature was measured
April 1978
OPTICS COMMUNICATIONS
Volume 25, number 1
Table 1 Cross section of absorption at wavelength of 337.1 nm oer, total decay rate y, radiative transition probability ficiency of fluorescence @P of POPOP, perylene and pyren in the vapor phase Q1
7
A
(lo-r7 cm’)
(lo9 s-‘)
(lo8 s-‘)
POPOP
8.0
5.7b
perylene pyren
0.45 0.60
1.17 (at 39OY) a 0.25 ’ 0.01
cl
0.70 0.011
A and quantum ef-
@F 0.49 (at 39O’C) a 0.28 0.11
a See fig. 2. b The value obtained in the liquid solution in ref. [9]. See text. c Ref. [8].
for a few tens hours. The coefficient of pyren is almost constant but coefficients of POPOP and perylene slightly decrease with time and the change during ten hours is about ten percent as shown in fig. 3. It was experimentally shown that this decrease did not result from a opaque on the surface of the glass. The rate of the thermal dissociation for the ground state of POPOP and perylene in the vapor phase at 350°C is estimated to be 0.007 and 0.010 per hour, respectively, from the decay curve in fig. 3. The experiment was made with the added buffer gases, N2 or He, up to buffer gas pressures of 400 torr. The addition of buffer gases within these pressures had no effect on the quantum efficiency and the thermal stability. It is concluded that thermal stabilities of POPOP, perylene and pyren at about 350°C are excellent and
much better than that of Rhodamine 6G in the vapor phase [I 61. The rate of the thermal dissociation of the molecule at about 350°C is very small and this process can be neglected during several tens of hours. The quantum efficiency of POPOP at temperatures below 300°C is roughly equal to the efficiency in the liquid solution and above 300°C decreases with temperatures. The efficiency at the temperature giving the peak output power, about 38O”C, is nearly equal to 0.5. One reason for the decrease in the power above 380°C is the temperature dependence of the quantum efficiency. On the other hand efficiencies of perylene and pyren which are independent of temperatures are much smaller than those in the solution. It may be evident that radiative transition probabilities of dyes in the vapor phase are nearly equal to those in the solution. The vapor density of dye molecules as a function of temperature is also obtained from the value of the absorption cross section in table 1 and the absorption coefficient with the cell in which a large amount of dye is inserted. The authors would like to express their thanks to Yukio Inoue for helpful discussions and also Masayoshi Ikeya for the technical assistance. Part of this work was supported by the Grant-in-Aid of the Scientific Research of the Ministry of Education.
References Fig. 3. Absorption coefficient of POPOP (a) and perylene (b) versus time at a constant temperature of 350°C.
[l] B. Steyer and F.P. Schafer, Appl. Phys. 7 (1975) 113. [2] N.A. Borisevich, Spectrosc. Lett. 8 (1975) 607.
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Volume 25, number 1
OPTICS COMMUNICATIONS
[3] L.C. Pikulik, V.A. Yakovenko and A.D. Das’ko, Zh. Prikl. Spektrosk. 23 (1975) 493. [4] P.W. Smith, Optica Acta 23 (1976) 901. (51 P.W. Smith, P.F. Liao, C.V. Shank, C. Kin and P.J. Maloney, IEEE J. Quantum Electron. QE-11 (1975) 84. [6] V.V. Gruzinskii, Bull. Acad. Sci. USSR Phys. Ser. (English transl.) 27 (1963) 576. [7] N.A. Borisevich, Bull. Acad. Sci. USSR Phys. Ser. (English transl.) 27 (1963) 559. [8] W.R. Ware and P.T. Cunningham, J. Chem. Phys. 44 (1966) 4364. [9] Catalog, Koch-Light Laboratories (England) p. 531.
78
April 1978
[lo] K. Kasper, Z. Physik Chem. 12 (1957) 55. [l l] B. Stevens, M.S. Walker and E. Hutton, Proc. Chem. Sot (1963) 62. [ 121 I.B. Berlman, Handbook of Fluorescence Spectra of Aromatic Molecules (Acad. Press, New York, 1965). [ 131 J.D. Laposa, E.C. Lim and R.E. Kellogg, J. Chem. Phys. 42 (1965) 3025. [14] C.A. Parker andT.A. Joyce, Chem. Comm. 1966) 234. [15] B. Stevens and B.E. Algar, Chem. Phys. Lett. 1 (1967) 219. [16] T. Sakurai and H.G. de Winter, J. Appl. Phys. 46 (1975) 875.
Takeki SAKURAI, Department
April 1978
OPTICS COMMUNICATIONS
Volume 25, number 1
OF ORGANIC DYE VAPORS
Atsushi OGISHIMA and Masao SUGAWARA
of Electronic
Engineering,
Faculty of Engineering,
Yamanashi University, Kofu, Japan
Received 28 November 1977 Organic dyes, POPOP, perylene and pyren, in the vapor phase are excited by a Nz laser. The quantum efficiency is measured as a function of temperature up to 45O’C. Optical properties such as the radiative transition probability are also estimated from the experimental results. It is found that the decrease in the vapor density by the thermal dissociation can be neglected during several tens of hours.
The vapor phase dye laser has been reported with various materials [I -41. Temperature is the main parameter to determine the power of this type of laser. Though the vapor pressure generally increases almost exponentially with the temperature, the power of the laser has a peak at some temperature. The dissociation of molecules and radiationless transition may be important at high temperature. These processes are deeply related with the quantum efficiency of fluorescence and the stability of the dye vapor. We report here detailed measurements of optical properties such as the quantum efficiency and the stability of POPOP, perylene and pyren in the vapor phase. POPOP is the most popular substance as a vaporphase dye laser and perylene and pyren, with which a laser oscillation is not obtained in the vapor phase, are examined in comparison with POPOP. The quantum efficiency of POPOP has been measured at temperatures below 330°C and reported to decrease with temperatures [5]. It is considered desirable to examine the change in the efficiency up to 450°C as described in this paper, since the peak of the laser power is obtained at about 380°C. In the case of perylene only a few data of optical properties in the vapor phase have been reported [6-81 and data are not enough to analyze an excitation or emission mechanism. Optical properties of pyren in the vapor phase are not well known. Crystals of dye molecules were inserted in a glass cilindrical cell with diameter of 2.0 cm and length of 2.5 cm, the cell was sealed off under vacuum and set in a heater box. Dye vapors in the cell were excited by
a N2 laser with wavelength of 337.1 nm and duration of 10 ns. A fluorescence from the dye near the entrance window for the exciting laser was detected by a photomultiplier after passing a monochromator. At the same time the absorption coefficient of the dye at 337.1 nm was measured. The amount of POPOP, for example, in the cell was so small that the density of the vapor was equal to 6.0 X 1015 cme3 when all of POPOP were evaporated. The amount was determined to keep the absorption coefficient almost constant at temperatures above 300°C. This makes it simple to measure the quantum efficiency up to 450°C. The cross section of absorption at 337.1 nm is also calculated from the constant absorption coefficient and the vapor density. In the experiment the intensity of the N2 laser was kept to be small enough not to saturate the ground state of the dye. The absorption coefficient and the fluorescence intensity of POPOP vapor measured at the peak of the fluorescence bands, 38.5 nm, and the peak of the duration are shown in fig. I. The lines shape was independent of the temperature. It is clear from fig. 1 that the quantum efficiency decreases with the temperature. Under the condition that the ground state of the dye is not saturated the rate equation describing the density of the first excited singlet state is easily solved, if the intensity profile of the pumping N2 laser, P, is given as P = PO sinot
O
P=O
n/o G t.
Ions, (1)
75
Volume
25. number
1
OPTICS COMMUNICATIONS
April 1978
iF~:--:::_ 0
+.
04:
250
300
350
400
TEMPERATURE
Fig. 1. Peak fluorescence intensity1 and absorption coefficient Q at 337.1 nm of POPOP vapor as a F unction of temperature. Vapor density above 350°C is 6.0 X 10” cme3.
When the peak density solved is denoted by F(y),,;n, the fluorescence intensity at the peak of duration, Ip, is shown by
$ = ~~ -we)
eY),,lX,,
(2)
where n is the total density of the dye, CYis the absorption coefficient at 337.1 nm,A is the radiative transition probability which is equal to the reciprocal of the natural life time, A, is the wavelength of the fluorescence, L(X,) is the normalized line shape, y is the total decay rate of the first excited state which is equal to the reciprocal of the fluorescence life time and k is a proportionality constant. The fluorescence intensity, the absorption coefficient and the line shape are measured from the experiment. Therefore, from the ratio of the fluorescence intensity of the dye vapor to POPOP in a liquid solution one unknown parameter A or y in the vapor phase can be estimated, because values of ys and As of POPOP in the liquid solution are known to be 6.7 X 1O8 s-l and 5.7 X I O8 s-l, respectively [9] . The quantum efficiency @P is given by A/y. The probability A of POPOP in the vapor phase is assumed to be equal to A, from the fact that the emission and the absorption curves show nearly perfect mirror symmetry and the absorption cross section integrated over the line shape is nearly equal to that in the solution. The decay rate and the quantum efficiency of POPOP estimated are shown in fig. 2 as a function of temperature. Values at temperatures below 300°C seem to agree with those in the solution which are denoted on the axis of ordinate in fig. 2. This is consistent with the results in ref. [S] . The efficiency at 450°C is almost one fourth of that in the solution. 76
Fig. 2. Total decay rate 7 and quantum cence @F of POPOP vapor as a function
450
(‘C)
efficiency of fluoresof temperature.
Fluorescence intensities of perylene and pyren at a constant vapor density were measured to be independent of temperatures within the experimental error. This indicates that the efficiency #F is constant and independent of temperature. It was verified from the line shape and the decay curve that observed emissions do not originate from a excimer [IO] and are not a delayed fluorescence [I I ] . The decay rate y of pyren was directly measured from the decay time of the fluorescence, which was much longer than the duration of the N, laser and the time resolution of the detector. It was confirmed from this direct measurement that y was independent of temperature. The decay rate of perylene in the vapor phase has been measured to be 2.5 X IO* s-l [8] . This value almost agreed with our result of the direct measurement under the consideration of the duration and the time resolution. Values of fundamental parameters obtained are listed in table 1. The radiative probability of perylene or pyren vapor roughly agrees with that in the solution [ 12- 151. The value A of perylene also agrees with the result in the vapor phase in ref. [8] but is four times as large as that in ref. [7]. The result in ref. [7] may be too small. The quantum efficiency of perylene is in fair agreement with the previous results in the vapor phase, 0.32 [6], 0.30 [7] and0.39 [8].H owever efficiencies of perylene and pyren in table 1 are considerably smaller than those in the solution [12-IS]. The thermal stability of the dye vapor is estimated from the time dependence of the absorption coefficient. When the cell temperature is so high that all of dye is evaporated, the absorption coefficient directly shows a change in the density of the ground state. The absorption coefficient at a constant temperature was measured
April 1978
OPTICS COMMUNICATIONS
Volume 25, number 1
Table 1 Cross section of absorption at wavelength of 337.1 nm oer, total decay rate y, radiative transition probability ficiency of fluorescence @P of POPOP, perylene and pyren in the vapor phase Q1
7
A
(lo-r7 cm’)
(lo9 s-‘)
(lo8 s-‘)
POPOP
8.0
5.7b
perylene pyren
0.45 0.60
1.17 (at 39OY) a 0.25 ’ 0.01
cl
0.70 0.011
A and quantum ef-
@F 0.49 (at 39O’C) a 0.28 0.11
a See fig. 2. b The value obtained in the liquid solution in ref. [9]. See text. c Ref. [8].
for a few tens hours. The coefficient of pyren is almost constant but coefficients of POPOP and perylene slightly decrease with time and the change during ten hours is about ten percent as shown in fig. 3. It was experimentally shown that this decrease did not result from a opaque on the surface of the glass. The rate of the thermal dissociation for the ground state of POPOP and perylene in the vapor phase at 350°C is estimated to be 0.007 and 0.010 per hour, respectively, from the decay curve in fig. 3. The experiment was made with the added buffer gases, N2 or He, up to buffer gas pressures of 400 torr. The addition of buffer gases within these pressures had no effect on the quantum efficiency and the thermal stability. It is concluded that thermal stabilities of POPOP, perylene and pyren at about 350°C are excellent and
much better than that of Rhodamine 6G in the vapor phase [I 61. The rate of the thermal dissociation of the molecule at about 350°C is very small and this process can be neglected during several tens of hours. The quantum efficiency of POPOP at temperatures below 300°C is roughly equal to the efficiency in the liquid solution and above 300°C decreases with temperatures. The efficiency at the temperature giving the peak output power, about 38O”C, is nearly equal to 0.5. One reason for the decrease in the power above 380°C is the temperature dependence of the quantum efficiency. On the other hand efficiencies of perylene and pyren which are independent of temperatures are much smaller than those in the solution. It may be evident that radiative transition probabilities of dyes in the vapor phase are nearly equal to those in the solution. The vapor density of dye molecules as a function of temperature is also obtained from the value of the absorption cross section in table 1 and the absorption coefficient with the cell in which a large amount of dye is inserted. The authors would like to express their thanks to Yukio Inoue for helpful discussions and also Masayoshi Ikeya for the technical assistance. Part of this work was supported by the Grant-in-Aid of the Scientific Research of the Ministry of Education.
References Fig. 3. Absorption coefficient of POPOP (a) and perylene (b) versus time at a constant temperature of 350°C.
[l] B. Steyer and F.P. Schafer, Appl. Phys. 7 (1975) 113. [2] N.A. Borisevich, Spectrosc. Lett. 8 (1975) 607.
77
Volume 25, number 1
OPTICS COMMUNICATIONS
[3] L.C. Pikulik, V.A. Yakovenko and A.D. Das’ko, Zh. Prikl. Spektrosk. 23 (1975) 493. [4] P.W. Smith, Optica Acta 23 (1976) 901. (51 P.W. Smith, P.F. Liao, C.V. Shank, C. Kin and P.J. Maloney, IEEE J. Quantum Electron. QE-11 (1975) 84. [6] V.V. Gruzinskii, Bull. Acad. Sci. USSR Phys. Ser. (English transl.) 27 (1963) 576. [7] N.A. Borisevich, Bull. Acad. Sci. USSR Phys. Ser. (English transl.) 27 (1963) 559. [8] W.R. Ware and P.T. Cunningham, J. Chem. Phys. 44 (1966) 4364. [9] Catalog, Koch-Light Laboratories (England) p. 531.
78
April 1978
[lo] K. Kasper, Z. Physik Chem. 12 (1957) 55. [l l] B. Stevens, M.S. Walker and E. Hutton, Proc. Chem. Sot (1963) 62. [ 121 I.B. Berlman, Handbook of Fluorescence Spectra of Aromatic Molecules (Acad. Press, New York, 1965). [ 131 J.D. Laposa, E.C. Lim and R.E. Kellogg, J. Chem. Phys. 42 (1965) 3025. [14] C.A. Parker andT.A. Joyce, Chem. Comm. 1966) 234. [15] B. Stevens and B.E. Algar, Chem. Phys. Lett. 1 (1967) 219. [16] T. Sakurai and H.G. de Winter, J. Appl. Phys. 46 (1975) 875.