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Radiative lifetime of nitrogen dioxide using a tunable organic dye laser
Volume 6. number 4
CHIZIICAL PHYSICSLETTERS
RADIATIVE USING
LIFETIME A TUNABLE
OF
NITROGEN
ORGANIC
DYE
15 August 1970
DIOXIDE LASER
PHILIP B. SACKETT * and JAMES T. YARDLEY Department of of Illinois. Urbana,Illinois 61801. USA Received 15 June 1970
The observation of time-resolved fluorescence from NOz molecules excited by a tunable organic dye laser (4400&49OOhi) has allowed a determination of the radiative lifetime as a function of excitation wavelength. Obsexwed lifetimes are in the range 70-82 ysec.
We have made direct time-resolved measurements of the radiative lifetime of electronically-excited NO2 molecules as a function of exciting wavelength over the range 44OOA-49OOA. These experiments, which utilize the high power, relative monochromaticity, and tunability of a ilashlamp-pumped organic dye laser, give information concerning the nature of radiative decay of electronically excited NO2, and demonstrate a powerful new technique for the study of the dynamics of excited molecular states. Radiative decay of NO2 excited in the visible
re-
gion of the spectrum is of particular interest since previous work has yielded lifetimes as much as two orders of magnitude longer than that estimated from the integrated absorption coefficient [ 1,2]. The recent measurements of Schwartz and Johnston [3] have shown the importance of measuring the variation of lifetime with excitation wavelength, although they used broad band excitation (25A) and the phase-shift method of lifetime determination. The organic dye laser used here is similar to that described by Sorokin et al. (41, with improvements recommended by Goldstein and Dacol [ 51. Wavelength selection is accomplished_with a diffraction grating (600 lines/mm, 5000A blaze) at one cud of the 65 cm cavity. The radiation is removed from the cavity with a dielectric mirror coated for 4-10% transmission over the range 4400-49OOA. The active medium is a flowing 2 X 10-4 M solution of 7-diethyl* National Science Foundation Graduate F’ellow 196% 1970.
; _--
amino-4-methylcoumarin in ethanoL (active length, 10 cm). The output consists of singleshot pulses with a base width of 0.5 @?ec and spectral base widths in the range I-4& In each experiment approximateLy 5% of the laser intensity is directed to a Spex ModeL 1’700 III Spectrograph for precise determination of the excitation wavelength; the remainder of the beam enters a 33 cm diameter spherical fLuorescence cell. An RCA 7265 photomultlplier tube (S-20 response) placed at right angLes to the Laser beam is used to detect the fluorescence. The field of view of the phototube is restricted to the central portion of the celL in order to observe only those regions where the probability that an excited molecule will reach the wall before fluorescing is very small [3]. Corning filters are used to reduce scattered laser radiation and to define the fluorescence wavelength regions observed. The signal from the phototube is dispLayed and photographed directly on a Tektronix Type 454 oscilloscope (5 hfRx bandwidth). The time base is calibrated with a precision Time Mark Generator. Lifetimes are normally determined by comparing the experimental curves to exponent&Is of known decay constants; maximum uncertainty in this me+&od is estiinated at 4
psec with the available signaL-to-noise. The NO2 samples are handled in a greaseless glass-metal system Fpith background pressures *2 X IO-7 tom. The samples are distiIIed from a reservoir of FarefulLy mn-ified NO2 held at 162°K; the vapor pressure of NO2 at this temperature is 04 x 10-5 torr. At this pressure the time betyeen collisions is about a factor of -40 323
Volume 6; number 4
CHEMICALPHYSICSLETTERS
greater than the.observed hecay times and the mean free path is a factor of six greater than the cell radius. Experiments on samples of NO2 distilled at 154’K showed no observable increase in lifetime. Sample impuritii?s,~on the other hand, were found to shorten-the lifetime markedly. Fig. 1 shows a plot of &served lifetime as a
function of excitation
wavelength using a Corning
2-61 filte:. which transmits wavelengths longer than 6100A. The lifetimes are seen to increase steadily from ~72 psec at 44OOA to ~80 psec at 49OOk If a Corning 3-69 filter (which passes wavelengths longer than 52nOA) is used, the curve maintains the sanie general trend. but is
lowered by approximately 6 psec. With either filter the lifetime spectrum contains local va-
riations too great to be attributed to experimental error. For instance. points A and B in fig. 1 represent lifetimes which differ by 2.4 psec. resulting_from excitation wavelengths differing by only 7A. Because of the extremely dense absorption -spectrum of N02. many indivjdual molecular states are excited by the I-4A wide laser pulses. Since the observed molecular decays are apparently exponential over the range 1 ~_csec-200 ysec. those-states which fluoresce most intensely in the observation region defined by the cut-off filter and the S-20 phototube response must have nearly the same radiative lifetime. Our directly-measured lifetimes (fig. 1, Corning 2-61 filter) iollow the same general trend with excitation wavelength as those obtained in the phase-shift measurements of Schwartz and Johnston (S-J), although ours are
15 August 1970
about 12 usec longer. This could be due to the higher No2 press&es used by S-J or to the dif-. ferent observation filters used (they used Corning 2-73. cut off at 58OOA). Schwartz and Jotinston’s zero-pressure Steorn-Volmer extrapolation ;or excit+tion at 480cA and observation at 6310..9, 661OA. and 7010A gives lifetimes of 81. 79. and SO psec, respectively. in quite good agreement with the values reported here with the Corning 2-61 observation filter. The variation of measured
lifetime
with observation
shooald be possible
5”
. : .-
.* .
-.. .
-*
.. .
* . -
I
monochromatic
.
I i
I
am
LSrn
1 LEm
urn -WAVELENGTH
EXCiTATlON
L8m
L?3l
61
1. .Lffetime spectrum; of NO2 excited by a tunable organic _dyeLaser. Pressure xP-x 1W5 torr; excitation b&e ti.dth. l-a, fluoresckzk observed through .’ Corning 2-61 filter (cut ofZ at 6lOOA). --.. Fig.
‘--
..
.
gives
clear
to extend the range of excita-
tion across the visible region of the spectrum and into the ultraviolet region [9,10]. Use of a Fabry-P&rot interferometer inside the laser cavity [ li] will make possible mu+ more highly
. .- *
region
evidence that moIecular states of different lifetimes are indeed excited during a single laser pulse. Qualitative explanations of anomalously long radiative lifetimes in small molecules such as NO2 have been discussed in detail by Douglas [6]. One of these explanations, which has been treated in a more quantitative manner by Bixon and Jortner [ 7.81. is that the Born-Oppenheimer states to which transitions from the ground state are allowed (those belonging to the 2B1 electronic state, according to S-J) are mixed with B-G states to which transitions from the ground state are forbidden (such as highly-excited vibrational states of the ground electronic state). Thus the lifetime is abnormally long by a factor dependent upon the magnitude of the off-diagonal matrix element which mixes the states and upon the density of zero-oscillator-strength states available for mixing. Since this factor would normally vary from one B-O state to another, variations in the lifetime spectrum of the mixed (stationary) s’ctes would be expected. The ability to observe these variations should be greatly dependent upon the line width of the exciting radiation. Using other dyes as active laser media. it
excitation
(=O.lA 1. thus allowing
for much finer resolution of the lifetime spectrum than is presented here. Thus further work can be expected to produce a much more detailed picture of radiative processes in NO2. Finally, it should be emphasized that the technique presented here is rattier general. Thus it is expected that fluor_escenbe sttidies utilizing tunable organic dye lasers iriill become an important new t-1 fqr the examinatio; of jntermOleCular energy transfer as‘ well as for the studi of ndiative processes. :, ._ : we giateftilly acknowledge support for this .._ -_ ._’ _-. ,. I_
Volume 6, number 4
CHEMICAL
work from the National Science Foundation, from the University of Illinois Research Board, and from E;I. du Pont de Nemours and Company. REFERENCES D. Neuberger and A. B. F. Duncan, J. Chem. Phys. 22 0954) 1693. [ZJ G. H.Myers. D.M.Silver and F. Kaufman, J. &em. Phys. 44 Q966) 718. [3] S. E. Schwartz and H. S. Johnston, J. Chem.Phys. 51 (1969) 1286; S. E. Schwartz, Ph. D. Thesis, University of Cab fornia. Berkeley (1968). [l]
PBYSICS
LETTERS
15 August 1970
141 V.L.Moruzzi and . . P. P. Sorokin. J. R. Lanka& E. C.Hsmmo~d. J. Chem. P&s. 48 (2968) 4726. 151 _ _ A.Goldstein and F. H. Dacol. Rev. Sci.Instr. 40 (1969) 1597. [Sj A.E. Do;&ia. J. Chem. Phys. 45 (1966) IOOC. [7] Y’X xon and J. Jortner. J. Chem. Phys. 48 (l968) [8] Fi8iixon
and J. Jortner. J. Chem. Phys. 50 (l969)
191J. A.*Myer. C. L. Johnson. E.K.ierstead. RD. Sbarma and I. Itzkan. Appi. Phys. Letters 16 (1970) [lo] k P. Broida and 5. C. Bkydon. Appl. Phys. Letters [ll]
16 (1970) 142.
D. J-Bradley. G.M. Gale. M. Moore and P. D. Smith. Pbys.Letters 26A (X966) 378.
325
CHIZIICAL PHYSICSLETTERS
RADIATIVE USING
LIFETIME A TUNABLE
OF
NITROGEN
ORGANIC
DYE
15 August 1970
DIOXIDE LASER
PHILIP B. SACKETT * and JAMES T. YARDLEY Department of of Illinois. Urbana,Illinois 61801. USA Received 15 June 1970
The observation of time-resolved fluorescence from NOz molecules excited by a tunable organic dye laser (4400&49OOhi) has allowed a determination of the radiative lifetime as a function of excitation wavelength. Obsexwed lifetimes are in the range 70-82 ysec.
We have made direct time-resolved measurements of the radiative lifetime of electronically-excited NO2 molecules as a function of exciting wavelength over the range 44OOA-49OOA. These experiments, which utilize the high power, relative monochromaticity, and tunability of a ilashlamp-pumped organic dye laser, give information concerning the nature of radiative decay of electronically excited NO2, and demonstrate a powerful new technique for the study of the dynamics of excited molecular states. Radiative decay of NO2 excited in the visible
re-
gion of the spectrum is of particular interest since previous work has yielded lifetimes as much as two orders of magnitude longer than that estimated from the integrated absorption coefficient [ 1,2]. The recent measurements of Schwartz and Johnston [3] have shown the importance of measuring the variation of lifetime with excitation wavelength, although they used broad band excitation (25A) and the phase-shift method of lifetime determination. The organic dye laser used here is similar to that described by Sorokin et al. (41, with improvements recommended by Goldstein and Dacol [ 51. Wavelength selection is accomplished_with a diffraction grating (600 lines/mm, 5000A blaze) at one cud of the 65 cm cavity. The radiation is removed from the cavity with a dielectric mirror coated for 4-10% transmission over the range 4400-49OOA. The active medium is a flowing 2 X 10-4 M solution of 7-diethyl* National Science Foundation Graduate F’ellow 196% 1970.
; _--
amino-4-methylcoumarin in ethanoL (active length, 10 cm). The output consists of singleshot pulses with a base width of 0.5 @?ec and spectral base widths in the range I-4& In each experiment approximateLy 5% of the laser intensity is directed to a Spex ModeL 1’700 III Spectrograph for precise determination of the excitation wavelength; the remainder of the beam enters a 33 cm diameter spherical fLuorescence cell. An RCA 7265 photomultlplier tube (S-20 response) placed at right angLes to the Laser beam is used to detect the fluorescence. The field of view of the phototube is restricted to the central portion of the celL in order to observe only those regions where the probability that an excited molecule will reach the wall before fluorescing is very small [3]. Corning filters are used to reduce scattered laser radiation and to define the fluorescence wavelength regions observed. The signal from the phototube is dispLayed and photographed directly on a Tektronix Type 454 oscilloscope (5 hfRx bandwidth). The time base is calibrated with a precision Time Mark Generator. Lifetimes are normally determined by comparing the experimental curves to exponent&Is of known decay constants; maximum uncertainty in this me+&od is estiinated at 4
psec with the available signaL-to-noise. The NO2 samples are handled in a greaseless glass-metal system Fpith background pressures *2 X IO-7 tom. The samples are distiIIed from a reservoir of FarefulLy mn-ified NO2 held at 162°K; the vapor pressure of NO2 at this temperature is 04 x 10-5 torr. At this pressure the time betyeen collisions is about a factor of -40 323
Volume 6; number 4
CHEMICALPHYSICSLETTERS
greater than the.observed hecay times and the mean free path is a factor of six greater than the cell radius. Experiments on samples of NO2 distilled at 154’K showed no observable increase in lifetime. Sample impuritii?s,~on the other hand, were found to shorten-the lifetime markedly. Fig. 1 shows a plot of &served lifetime as a
function of excitation
wavelength using a Corning
2-61 filte:. which transmits wavelengths longer than 6100A. The lifetimes are seen to increase steadily from ~72 psec at 44OOA to ~80 psec at 49OOk If a Corning 3-69 filter (which passes wavelengths longer than 52nOA) is used, the curve maintains the sanie general trend. but is
lowered by approximately 6 psec. With either filter the lifetime spectrum contains local va-
riations too great to be attributed to experimental error. For instance. points A and B in fig. 1 represent lifetimes which differ by 2.4 psec. resulting_from excitation wavelengths differing by only 7A. Because of the extremely dense absorption -spectrum of N02. many indivjdual molecular states are excited by the I-4A wide laser pulses. Since the observed molecular decays are apparently exponential over the range 1 ~_csec-200 ysec. those-states which fluoresce most intensely in the observation region defined by the cut-off filter and the S-20 phototube response must have nearly the same radiative lifetime. Our directly-measured lifetimes (fig. 1, Corning 2-61 filter) iollow the same general trend with excitation wavelength as those obtained in the phase-shift measurements of Schwartz and Johnston (S-J), although ours are
15 August 1970
about 12 usec longer. This could be due to the higher No2 press&es used by S-J or to the dif-. ferent observation filters used (they used Corning 2-73. cut off at 58OOA). Schwartz and Jotinston’s zero-pressure Steorn-Volmer extrapolation ;or excit+tion at 480cA and observation at 6310..9, 661OA. and 7010A gives lifetimes of 81. 79. and SO psec, respectively. in quite good agreement with the values reported here with the Corning 2-61 observation filter. The variation of measured
lifetime
with observation
shooald be possible
5”
. : .-
.* .
-.. .
-*
.. .
* . -
I
monochromatic
.
I i
I
am
LSrn
1 LEm
urn -WAVELENGTH
EXCiTATlON
L8m
L?3l
61
1. .Lffetime spectrum; of NO2 excited by a tunable organic _dyeLaser. Pressure xP-x 1W5 torr; excitation b&e ti.dth. l-a, fluoresckzk observed through .’ Corning 2-61 filter (cut ofZ at 6lOOA). --.. Fig.
‘--
..
.
gives
clear
to extend the range of excita-
tion across the visible region of the spectrum and into the ultraviolet region [9,10]. Use of a Fabry-P&rot interferometer inside the laser cavity [ li] will make possible mu+ more highly
. .- *
region
evidence that moIecular states of different lifetimes are indeed excited during a single laser pulse. Qualitative explanations of anomalously long radiative lifetimes in small molecules such as NO2 have been discussed in detail by Douglas [6]. One of these explanations, which has been treated in a more quantitative manner by Bixon and Jortner [ 7.81. is that the Born-Oppenheimer states to which transitions from the ground state are allowed (those belonging to the 2B1 electronic state, according to S-J) are mixed with B-G states to which transitions from the ground state are forbidden (such as highly-excited vibrational states of the ground electronic state). Thus the lifetime is abnormally long by a factor dependent upon the magnitude of the off-diagonal matrix element which mixes the states and upon the density of zero-oscillator-strength states available for mixing. Since this factor would normally vary from one B-O state to another, variations in the lifetime spectrum of the mixed (stationary) s’ctes would be expected. The ability to observe these variations should be greatly dependent upon the line width of the exciting radiation. Using other dyes as active laser media. it
excitation
(=O.lA 1. thus allowing
for much finer resolution of the lifetime spectrum than is presented here. Thus further work can be expected to produce a much more detailed picture of radiative processes in NO2. Finally, it should be emphasized that the technique presented here is rattier general. Thus it is expected that fluor_escenbe sttidies utilizing tunable organic dye lasers iriill become an important new t-1 fqr the examinatio; of jntermOleCular energy transfer as‘ well as for the studi of ndiative processes. :, ._ : we giateftilly acknowledge support for this .._ -_ ._’ _-. ,. I_
Volume 6, number 4
CHEMICAL
work from the National Science Foundation, from the University of Illinois Research Board, and from E;I. du Pont de Nemours and Company. REFERENCES D. Neuberger and A. B. F. Duncan, J. Chem. Phys. 22 0954) 1693. [ZJ G. H.Myers. D.M.Silver and F. Kaufman, J. &em. Phys. 44 Q966) 718. [3] S. E. Schwartz and H. S. Johnston, J. Chem.Phys. 51 (1969) 1286; S. E. Schwartz, Ph. D. Thesis, University of Cab fornia. Berkeley (1968). [l]
PBYSICS
LETTERS
15 August 1970
141 V.L.Moruzzi and . . P. P. Sorokin. J. R. Lanka& E. C.Hsmmo~d. J. Chem. P&s. 48 (2968) 4726. 151 _ _ A.Goldstein and F. H. Dacol. Rev. Sci.Instr. 40 (1969) 1597. [Sj A.E. Do;&ia. J. Chem. Phys. 45 (1966) IOOC. [7] Y’X xon and J. Jortner. J. Chem. Phys. 48 (l968) [8] Fi8iixon
and J. Jortner. J. Chem. Phys. 50 (l969)
191J. A.*Myer. C. L. Johnson. E.K.ierstead. RD. Sbarma and I. Itzkan. Appi. Phys. Letters 16 (1970) [lo] k P. Broida and 5. C. Bkydon. Appl. Phys. Letters [ll]
16 (1970) 142.
D. J-Bradley. G.M. Gale. M. Moore and P. D. Smith. Pbys.Letters 26A (X966) 378.
325