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Fluorescence of perylene in crystalline n-heptane, selectively laser excited in the phonon sideband
Journal of Luminescence 24/25 (1981)655—658 North-Holland Publishing Company
655
FLUORESCENCE OF PERYLENE IN CRYSTALLINE n-HEPTANE, SELECTIVELY LASER EXCITEO IN THE PHONON SIDEBANO U. Bogner, B. Plail
and H. Tauschek
Institut Physik III, Universitat Regensburg 0-8400 Regensburg Fed. Rep. Germany
Perylene in crystalline n-heptane has been selectively excited by a pulsed narrow-band dye-laser in the phonon sideband of the pure electronic and the first vibronic transition at 1.2 K. Depending on the laser wavelength “1 me splitting” with two or three lines or a fluorescence signal in the anti-Stokes region of the zero-phonon lines or stimulated emission of zero—phonon lines is obtained. The results are explained by electron-phonon-interaction involving pseudolocalized phonon modes and by site selection via excitation in the phonon sideband. The fluorescence spectra of the aromatic hydrocarbon perylene in crystalline n-paraffins have been extensively studied at low temperatures because its linear electron-phonon interaction (EPI) is strong, as compared to that of other dye molecules showing quasiline spectra in Shpol ‘skii matrices [1 — 3]. Recently selectively laser-excited perylene, matrix-isolated in Langmuir films provided new optical methods for phonon detection in the anti-Stokes phonon side-band and by the phonon memory [4]demonstrating that there exists not only EPI but in addition a direct interaction of phonons with the dye-matrix system. This interaction is explained in a model [41by photon- and phonon-induced transitions in localized two-level systems in amorphous matrices. In order to separate the effects of this additional interactioVIrom the effects of the EPI, which is also present in amorphous matrices, it is interesting, to study the EPI isolatedly in the crystalline matrix, in particular it is interesting to obtain information on the physical nature of the phonon sideband, e. g., by selective laser excitation in the phonon sideband. These investigations yielded results, which concern also general questions of site selection spectroscopy and fluorescence line narrowing in the case of strong EPI and aspects of stimulated emission of zero-phonon lines and the generation of monochromatic phonons [51. In the present paper main emphasis is given to spectroscopic results, which concern in particular “line splitting” with two or three lines in the fluorescence region of the zero-phonon lines, obtained at certain excitation wavelengths. The Shpol’skii matrix n-heptane is elected because the linear EPI of perylene in n-heptane [2] is stronger than that in other n-paraffins e. g. n-octane [3]. A Debye-Waller factor cs= 0.23 was found [2] for the pure electronic transition of site I which has its (0+0) zero-phonon line at the wavelength X= 4408 ~. The sample of n-heptane (Uvasol grade) with perylene was prepared in a quartz vessel, sealed at the temperature of liquid N 2. The perylene concentration was typical 4 10-6 mol/l. The sample was immersed at 1.2 K in superfluid helium in an optical cryostat. The sample was excited by a flash-lamp pumped dye-laser (Coumarin 120 in methanol; peak power~3kW; pulse width ~0.7 ps; line width <0.1 A). The spectra were recorded by an optical multichannel analyzer with expanding optic at the exit side of a 0.85 m double monochromator. The use of the multichannel analyzer eliminates the problem of repeatability (%0.4 A) of 0 022—2313/81/0000—0000/502.75 © Norlh-Holland
656
F. Bogitc’r St aS. / Huorcsce,u~c(,/perileiic in crvvtalline n—hcplanc
~L25 ~4O8.8A:::*3~12~5 ~
4408.5A
\i.i~5
44066 A
z /.1.07.9A 2
2
~ ..._..._.-_~\J
~o6~4A
~7A2 ~-A(A)4480
4/.75
..-xl$3 4/.80
4475
2L Fig. 1. Fluorescence spectra of Perylene in n-heptane at 1.2 K in the region of the flash-lamp pumped dye-laser. the exciting first vibronic zero-phonon line of site I at different wavelengths the monochromator, therefore line shifts <0.1 A can be taken from successive fluorescence spectra. The fluorescence spectra in the region of the first vibronic zero-phonon line (0.1) of site I are shown in Fig. 1. At the laser wavelength ~L= 4408.8 A the spectrum shows at 4473.g A an intensive zero—phonon line and a phonon sideband with a ma ilum shifted 16 cur1 to longer wavelengths. Tuning the laser wavelength to shorter wavelengths the fluorescence intensity is considerably reduced and ir 1 is shifted. At >[= 4407.9 A a second line (line number 2 in Fig. 1) c o~s t 4478.9 A. At 2[= 4407.7 A line 2 increases while line 1 is further reducu ~L=4407.4 A a third line appears at the long wavelengths side of line 2~ s line 3 becomes more prominent as the laser is shifted from XL= 4407.2 A ~ 4406.9 A. At ~[= 4406.9 A and ~L= 4406.4 A line 1 becomes very we ,hile line 3 coincides with line 2. The explanatior. of the line splitting can be taken from Fig. 2, showing the schematic diagram of the absorption spectrum of the pure electronic transition (0+0) and the emission spectrum of the first vibronic transition (0÷1). The situation shown in this diagram is similar to that in the case of the wavelength ~L= 4407.4 A (see Fig. 1) of the exciting dye laser. In this case the narrow—band dye laser excites at the perylene molecules PM 1 in the zero phonon line of their absorption spectrum (solid curve of the absorption diagram in Fig. 2 ). These molecules PM1 yield a separate contribution to the fluorescence spectrum in the emission of the first vibronic transition. This contribution has the spectral shape of the solid line of the emission diagram in Fig. 2. At the wavelength )Lthere is also the absorption (dotted curve of
U. Bogner et aL
/ Fluorescence of perylcne in crystalline n-heptane
/
ABSORPTION
657
/PM~
‘,
(,~
PM
3!
/
AL
~
2
EMISSION
—A
/
A3 A2 A1
Fig. 2. Schematic diagram of the absorption spectrum of the pure elektronic transition and the emission spectrum of the first vibronic transition. the absorption diagram) of dye molecules PM2 which absorb at X[ in their phonon sideband. The absorption in the phonon sideband is small but the occupation of these levels with perylene molecules is much higher than that at the wavelength XL_ in particular in the center of the inhomogenously broadened zero-phonon line of the total absorption spectrum (dashed-dotted line of the absorption diagram). The total contribution 12 (0) of the zero-phonon lines of all molecules PM2 to the emission spectrum at the frequency v is given by: 12(01
=
(1—
ccA)csE
f
N(v’)ah(vL—v’)
f0(v—o’)
do’
°A is the Debye—Waller factor of the (D~-D)transition of the absorption spectrum, °E is the Debye-Waller factor of the (0÷1) transition of the emission spectrum, a h&L-°) is the probability for the absorption of a laser photon with freque~cy°L in the level E’ = h~i, f0 (o-~i) is the probability for the emission of a photon with frequency v from the level E’ = ho’, N(v’) is the occupation of the levels E’ = ho’ with perylene molecules. While line 1 is shifted with the laser wavelength, line 2 is bound to the site distribution to line with 1. There are two differentXL,contributions N(v’). Line 3 at X3 is also1 shifted the laser wavelength it has a to line 3.distance The first is given by the zero-phonon line of perylene molecules constant of one 16 cm PM 3 excited at XL in the sharp maximum of the phonon sideband in their absorption spectrum (dashed curve of the absorption diagram). The second contribution to line 3 is given by the sharp maximum in the phonon-sideband of the emission spectrum of PM1 (solid curve of the emission diagram). At XL = 4406.9 A, the contribution of the zero-phonon lines of PM3 is dominant since at this XL the mxci tationof PM1 is too weak - see line 1 in the spectrum for XL = 4406.9 A in Fig. 1 — to yimld a strong contribution. We attribute the sharp maximum in the phonon sideband to pseudolocalized [61 phonon modes and not to a maximum [21 in the phonon density of states of the dalocalized phonon modes. This assumption is confirmed by the fact that no maximum at 16 cm~ is obtained in the measurement of the density of states
658
U Bogner St aS. / Fluorescence ofpen lene in crrstal/ine n./reptane
of delocalized phonons in n—heptane by neutron scattering [7]. The assumption is also confirmed by the following spectroscopic observations. If the exciting laser wavelength XL is chosen shorter than 4406 A the fluorescence signal in the anti—Stokes region of the (0÷1)zero-phonon line increases once more, achieving its maximum at ~L = 4405.5 A; in this case the anti—Stokes signal has the same distance to the zero-phonon line than the maximum in the phonon sideband at the Stokes side. The results of the investigation of this antiStokes signal, in particular its dependence on the concentration of perylene, confirm the above—mentioned assumption, and demonstrate the generation of monochromatic phonons [5]. These investigations at high concentrations of perylene (o5 . 10 niol) also demonstrated that there is stimulated emission of zero—phonon lines not only in the case of excitation in the zero-phonon lines of the absorption spectrum, but also in the case of excitation in the phonon side bands [5] 5 mol/l the stimulated emission If the perylene concentration less thanof iOcould be recognized by a nonlinearis increase the intensity of the first vibronic and the pure electronic zero-phonon lines only if the excitation wavelength >[ is chosen coincident with the center of the zero—phonon lines of the absorption spectrum. But in the case of the excitation in the phononsideband of the absorption spectrum the dependence of the Debye-Waller factor on the wavelength and on the power density of the exciting laser yields indication of the beginning of stimulated emission also at low perylene concentrations. The line splitting, the anti—Stokes fluorescence signal and the stimulated emission are also observed in the case of excitation in the phonon sideband of the (1X-O) zero-phonon line. In this case the effects can be observed also in the region of the (0.0) transition, but the fluorescence spectra are affected by reabsorption. ACKNOWL EDGEMENTS The authors wish to thank Professors M. Maier and M. Creuzburg for helpful discussions. Thanks are also due to Dr. W. Press, for the measurement of the phonon density of states of n-heptane by neutron scattering. REFERENCES: [1] I. S. Osadko, P. I. Personov and E. V. Shpol skii, U. Luminescence 6 (1973) 369. [2] R. I. Personov, I. S. Osadko, E. D. Godyaev and F. I. Al ‘shits, Soy. Phys. Solid State 13 (1972) 2224. [3] F. I. Al shits, E. 0. Godyaev and R. I. Personov, Soy. Phys. Solid State 14 (1972) 2224. [4]U. Bogner, Phys. Rev. Lett. 37 (1976) 909. [51 U. Bogner, B. Plail and H. Tauschek, in preparation. [6] see, e.g.,P. H. Chereson. P. S. Friedman and R. Kopelman, 3. Chem. Phys. 56 (1972) 3716. [7] W. Press, private communication.
655
FLUORESCENCE OF PERYLENE IN CRYSTALLINE n-HEPTANE, SELECTIVELY LASER EXCITEO IN THE PHONON SIDEBANO U. Bogner, B. Plail
and H. Tauschek
Institut Physik III, Universitat Regensburg 0-8400 Regensburg Fed. Rep. Germany
Perylene in crystalline n-heptane has been selectively excited by a pulsed narrow-band dye-laser in the phonon sideband of the pure electronic and the first vibronic transition at 1.2 K. Depending on the laser wavelength “1 me splitting” with two or three lines or a fluorescence signal in the anti-Stokes region of the zero-phonon lines or stimulated emission of zero—phonon lines is obtained. The results are explained by electron-phonon-interaction involving pseudolocalized phonon modes and by site selection via excitation in the phonon sideband. The fluorescence spectra of the aromatic hydrocarbon perylene in crystalline n-paraffins have been extensively studied at low temperatures because its linear electron-phonon interaction (EPI) is strong, as compared to that of other dye molecules showing quasiline spectra in Shpol ‘skii matrices [1 — 3]. Recently selectively laser-excited perylene, matrix-isolated in Langmuir films provided new optical methods for phonon detection in the anti-Stokes phonon side-band and by the phonon memory [4]demonstrating that there exists not only EPI but in addition a direct interaction of phonons with the dye-matrix system. This interaction is explained in a model [41by photon- and phonon-induced transitions in localized two-level systems in amorphous matrices. In order to separate the effects of this additional interactioVIrom the effects of the EPI, which is also present in amorphous matrices, it is interesting, to study the EPI isolatedly in the crystalline matrix, in particular it is interesting to obtain information on the physical nature of the phonon sideband, e. g., by selective laser excitation in the phonon sideband. These investigations yielded results, which concern also general questions of site selection spectroscopy and fluorescence line narrowing in the case of strong EPI and aspects of stimulated emission of zero-phonon lines and the generation of monochromatic phonons [51. In the present paper main emphasis is given to spectroscopic results, which concern in particular “line splitting” with two or three lines in the fluorescence region of the zero-phonon lines, obtained at certain excitation wavelengths. The Shpol’skii matrix n-heptane is elected because the linear EPI of perylene in n-heptane [2] is stronger than that in other n-paraffins e. g. n-octane [3]. A Debye-Waller factor cs= 0.23 was found [2] for the pure electronic transition of site I which has its (0+0) zero-phonon line at the wavelength X= 4408 ~. The sample of n-heptane (Uvasol grade) with perylene was prepared in a quartz vessel, sealed at the temperature of liquid N 2. The perylene concentration was typical 4 10-6 mol/l. The sample was immersed at 1.2 K in superfluid helium in an optical cryostat. The sample was excited by a flash-lamp pumped dye-laser (Coumarin 120 in methanol; peak power~3kW; pulse width ~0.7 ps; line width <0.1 A). The spectra were recorded by an optical multichannel analyzer with expanding optic at the exit side of a 0.85 m double monochromator. The use of the multichannel analyzer eliminates the problem of repeatability (%0.4 A) of 0 022—2313/81/0000—0000/502.75 © Norlh-Holland
656
F. Bogitc’r St aS. / Huorcsce,u~c(,/perileiic in crvvtalline n—hcplanc
~L25 ~4O8.8A:::*3~12~5 ~
4408.5A
\i.i~5
44066 A
z /.1.07.9A 2
2
~ ..._..._.-_~\J
~o6~4A
~7A2 ~-A(A)4480
4/.75
..-xl$3 4/.80
4475
2L Fig. 1. Fluorescence spectra of Perylene in n-heptane at 1.2 K in the region of the flash-lamp pumped dye-laser. the exciting first vibronic zero-phonon line of site I at different wavelengths the monochromator, therefore line shifts <0.1 A can be taken from successive fluorescence spectra. The fluorescence spectra in the region of the first vibronic zero-phonon line (0.1) of site I are shown in Fig. 1. At the laser wavelength ~L= 4408.8 A the spectrum shows at 4473.g A an intensive zero—phonon line and a phonon sideband with a ma ilum shifted 16 cur1 to longer wavelengths. Tuning the laser wavelength to shorter wavelengths the fluorescence intensity is considerably reduced and ir 1 is shifted. At >[= 4407.9 A a second line (line number 2 in Fig. 1) c o~s t 4478.9 A. At 2[= 4407.7 A line 2 increases while line 1 is further reducu ~L=4407.4 A a third line appears at the long wavelengths side of line 2~ s line 3 becomes more prominent as the laser is shifted from XL= 4407.2 A ~ 4406.9 A. At ~[= 4406.9 A and ~L= 4406.4 A line 1 becomes very we ,hile line 3 coincides with line 2. The explanatior. of the line splitting can be taken from Fig. 2, showing the schematic diagram of the absorption spectrum of the pure electronic transition (0+0) and the emission spectrum of the first vibronic transition (0÷1). The situation shown in this diagram is similar to that in the case of the wavelength ~L= 4407.4 A (see Fig. 1) of the exciting dye laser. In this case the narrow—band dye laser excites at the perylene molecules PM 1 in the zero phonon line of their absorption spectrum (solid curve of the absorption diagram in Fig. 2 ). These molecules PM1 yield a separate contribution to the fluorescence spectrum in the emission of the first vibronic transition. This contribution has the spectral shape of the solid line of the emission diagram in Fig. 2. At the wavelength )Lthere is also the absorption (dotted curve of
U. Bogner et aL
/ Fluorescence of perylcne in crystalline n-heptane
/
ABSORPTION
657
/PM~
‘,
(,~
PM
3!
/
AL
~
2
EMISSION
—A
/
A3 A2 A1
Fig. 2. Schematic diagram of the absorption spectrum of the pure elektronic transition and the emission spectrum of the first vibronic transition. the absorption diagram) of dye molecules PM2 which absorb at X[ in their phonon sideband. The absorption in the phonon sideband is small but the occupation of these levels with perylene molecules is much higher than that at the wavelength XL_ in particular in the center of the inhomogenously broadened zero-phonon line of the total absorption spectrum (dashed-dotted line of the absorption diagram). The total contribution 12 (0) of the zero-phonon lines of all molecules PM2 to the emission spectrum at the frequency v is given by: 12(01
=
(1—
ccA)csE
f
N(v’)ah(vL—v’)
f0(v—o’)
do’
°A is the Debye—Waller factor of the (D~-D)transition of the absorption spectrum, °E is the Debye-Waller factor of the (0÷1) transition of the emission spectrum, a h&L-°) is the probability for the absorption of a laser photon with freque~cy°L in the level E’ = h~i, f0 (o-~i) is the probability for the emission of a photon with frequency v from the level E’ = ho’, N(v’) is the occupation of the levels E’ = ho’ with perylene molecules. While line 1 is shifted with the laser wavelength, line 2 is bound to the site distribution to line with 1. There are two differentXL,contributions N(v’). Line 3 at X3 is also1 shifted the laser wavelength it has a to line 3.distance The first is given by the zero-phonon line of perylene molecules constant of one 16 cm PM 3 excited at XL in the sharp maximum of the phonon sideband in their absorption spectrum (dashed curve of the absorption diagram). The second contribution to line 3 is given by the sharp maximum in the phonon-sideband of the emission spectrum of PM1 (solid curve of the emission diagram). At XL = 4406.9 A, the contribution of the zero-phonon lines of PM3 is dominant since at this XL the mxci tationof PM1 is too weak - see line 1 in the spectrum for XL = 4406.9 A in Fig. 1 — to yimld a strong contribution. We attribute the sharp maximum in the phonon sideband to pseudolocalized [61 phonon modes and not to a maximum [21 in the phonon density of states of the dalocalized phonon modes. This assumption is confirmed by the fact that no maximum at 16 cm~ is obtained in the measurement of the density of states
658
U Bogner St aS. / Fluorescence ofpen lene in crrstal/ine n./reptane
of delocalized phonons in n—heptane by neutron scattering [7]. The assumption is also confirmed by the following spectroscopic observations. If the exciting laser wavelength XL is chosen shorter than 4406 A the fluorescence signal in the anti—Stokes region of the (0÷1)zero-phonon line increases once more, achieving its maximum at ~L = 4405.5 A; in this case the anti—Stokes signal has the same distance to the zero-phonon line than the maximum in the phonon sideband at the Stokes side. The results of the investigation of this antiStokes signal, in particular its dependence on the concentration of perylene, confirm the above—mentioned assumption, and demonstrate the generation of monochromatic phonons [5]. These investigations at high concentrations of perylene (o5 . 10 niol) also demonstrated that there is stimulated emission of zero—phonon lines not only in the case of excitation in the zero-phonon lines of the absorption spectrum, but also in the case of excitation in the phonon side bands [5] 5 mol/l the stimulated emission If the perylene concentration less thanof iOcould be recognized by a nonlinearis increase the intensity of the first vibronic and the pure electronic zero-phonon lines only if the excitation wavelength >[ is chosen coincident with the center of the zero—phonon lines of the absorption spectrum. But in the case of the excitation in the phononsideband of the absorption spectrum the dependence of the Debye-Waller factor on the wavelength and on the power density of the exciting laser yields indication of the beginning of stimulated emission also at low perylene concentrations. The line splitting, the anti—Stokes fluorescence signal and the stimulated emission are also observed in the case of excitation in the phonon sideband of the (1X-O) zero-phonon line. In this case the effects can be observed also in the region of the (0.0) transition, but the fluorescence spectra are affected by reabsorption. ACKNOWL EDGEMENTS The authors wish to thank Professors M. Maier and M. Creuzburg for helpful discussions. Thanks are also due to Dr. W. Press, for the measurement of the phonon density of states of n-heptane by neutron scattering. REFERENCES: [1] I. S. Osadko, P. I. Personov and E. V. Shpol skii, U. Luminescence 6 (1973) 369. [2] R. I. Personov, I. S. Osadko, E. D. Godyaev and F. I. Al ‘shits, Soy. Phys. Solid State 13 (1972) 2224. [3] F. I. Al shits, E. 0. Godyaev and R. I. Personov, Soy. Phys. Solid State 14 (1972) 2224. [4]U. Bogner, Phys. Rev. Lett. 37 (1976) 909. [51 U. Bogner, B. Plail and H. Tauschek, in preparation. [6] see, e.g.,P. H. Chereson. P. S. Friedman and R. Kopelman, 3. Chem. Phys. 56 (1972) 3716. [7] W. Press, private communication.