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The hyperfine structure of molecular iodine in saturated fluorescence
Volume 25, number 1
OPTICS COMMUNICATIONS
THE HYPERFINE STRUCTURE
OF MOLECULAR
IODINE IN SATURATED
April 1978
FLUORESCENCE
E. DOPEL, D. KUHLKE and W. DIETEL Friedrich-Schiller-Universitiit
Jena, Sektion Physik, DDR-69 Jena
Received 3 January 1978
The hyperfine structure of the R(127) 11-5 line (633 nm) of ‘*‘I2 were investigated by means of the saturated fluorescence excited in the resonator of a single mode 3He-20 Ne laser. Fourteen hyperfine components were detected in the tuning range of the laser at pressures from 15 to 240 mtorr.
The hyperfine structure (hfs) of the 12712-molecule (R( 127) line in the 1 l-5 band of the B 311(Oz) - X 1 Z(Oi) electronic transition) was first observed by Hanes and Dahlstrom by means of Doppler-free saturated absorption spectroscopy [l] . They found small narrow peaks from the saturated intracavity absorption of this hfs-components in the output of a single mode 3 He-20Ne laser with a wavelength of 633 mn. We report here for the first time the observation of these hfs-components by means of saturated fluorescence. This method was suggested by Basov and Letokhov [2] and verificated experimentally by Freed and Javan [3] on C02-molecules and by Sorem and Schawlow [4] on 12-molecules in the 5 14 nm region. The technique of the saturated fluorescence is also a method of Doppler-free spectroscopy. In difference to the saturated absorption spectroscopy where the absorption saturated by the counterrunning waves of a monochromatic field is measured in this method the saturated population of a level excited by a monochromatic standing wave field is measured. When tuning the field frequency to the centre frequency of the transition a relative minimum appears in the fluorescence from this excited level. The halfwidth of this dip is determined by the natural width of the exciting transition, Compared with the saturated absorption spectroscopy the method of saturated fluorescence offers several advantages. Since the fluorescence of the molecule is detected the strong laser background is eliminated. Therefore with this method a sensitive 62
saturation spectroscopy of dilute gases of a very low absorption can be performed [3-61. Further this technique is very suitable for the investigation of molecular transitions. Relaxation processes between the vibrational-rotational sublevels of the excited electronic state due to the collisions and velocity changes of the molecules caused also by such collisions diminish the contrast of the absorption dips in the case of saturated absorption spectroscopy [lo] . In the case of saturated fluorescence spectroscopy the contrast is decreased only due to the increasing saturation intensity since the fluorescence from all the vibrational-rotational sublevels of the electronic state is detected [S, 71. In a theoretical treatment the influence of these processes and of the spatially inhomogeneous population distribution of the molecules due to the standing wave pattern on contrast and shape of the fluorescence dips was investigated [7]. In tig. 1 the experimental setup is shown. The iodine cell is placed inside the resonator of a strong single mode He-Ne laser. Single mode operation was achieved by the method of nonlinear absorption [8]. With a He-Ne laser having an active length of 1.70 m and a Ne-tube as nonlinear absorber a power of 35 mW was achieved in single mode operation with an output mirror of R = 96%. However, in order to obtain a high intracavity power in these experiments mirrors with a reflection coefficient >99% were used. The high single mode power and the continuous changing of the laser frequency over more than 500 MHz was achieved by a considerable contribution of homogeneous of the
Volume 25, number 1
April 1978
OPTICS COMMUNICATIONS
abc
hij
defg
klmn cb)
Fig. 1. Schematic drawing of the experimental setup. PM photomultiplier, PZT piezoelectric transducer, SML single mode laser, PC peltier cooler, L lens, S slit, F filter, SIN-GEN sinus generator, RAMP-GEN ramp generator.
laser gain profile [9] . The fluorescing beam inside the quartz iodine cell was imaged with a lens (L) on a slit (S). For registration with the photomultiplier (PM) the antistokes fluorescence was selected with a 600 run interference filter (F). The photomultiplier has to be shielded carefully against scattered light of the laser beam. The vapour pressure of 12 is controled with a peltier cooler (PC). The frequency of the laser beam was periodically shifted by 4 MHz by applying a 500 Hz sinusoidal ac-voltage (SIN-GEN) to a piezoelectric transducer (PZT) which carries one of the end mirrors. The signal was detected with a phase sensitive amplifier. When the laser frequency was tuned slowly over the line first derivative of the hfs-components was obtained. For this tuning a second PZT-element or the thermal laser drift was used. The frequency position of the single mode was controled by optical heterodyning with a wobbeled single mode laser (SML). Figs. 2a and b show typical recorder traces with the fourteen hyperfine components of 127I2 also reported by Hanes and Dahlstrom in saturated absorption. The hfs-components were recorded in the pressure region from 15 to 240 mtorr with an intracavity laser power of about 1 W/cm2. In this pressure region linewidths from 5 to 10 MHz were obtained. We wish to acknowledge the support of Professor V.S. Letokhov and helpful discussions with Professor G. Hesse.
defg
hij
klmn
Fig. 2. Typical recorder traces. (a) Recorded with the following experimental parameters: tuning velocity 1.5 MHz/s, integration constant Is, iodine pressure 170 mtorr, amplitude of frequency modulation 4 MHz, intracavity laser intensity about 1 W/cm’. (b) Recorded with a tuning velocity of 1 MHz/s and a integration constant of 3 s. The well known spacing of the h-i and i-j components of 22 MHz [l] may serve as a frequency scale.
References [l] G.R. Hanes and C.E. Dahlstrom, Appl. Phys. Lett. 14 (1969) 326. [ 21 N.G. Basov and V.S. Letokhov, Report on URSI Conference “Laser Measurements” Sept. 1968 Warsaw, Poland. [3] C. Freed and A. Javan, Appl. Phys. Lett. 17 (1970) 53. [4] M.S. Sorem and A. Schawlow, Opt. Comm. 5 (1972) 148.
63
Volume 25, number 1
OPTICS COMMUNICATIONS
[S] V.S. Letokhov and B.D. Pavlik, Zh. Eksp. i Tear. Fiz. 64 (1973) 804. [6] K. Shimoda, Appl. Phys. 1 (1973) 77. [ 71 E. Diipel and D. Kiihlke, Czech. Journ. Phys., in press. [8] V.P. Chebotayev, LM. Beterov and V.N. Lisitsyn, IEEE J. Quant. Electr. QE (1973) 788.
64
April 1978
[9] W. Dietel, D. Kiihlke and H. Sander, Opt. and Quant. Electr. 7 (1975) 345. [lo] D. Kiihlke and E. DGpel, Czech. Journ. Phys. B25 (1975) 1084.
OPTICS COMMUNICATIONS
THE HYPERFINE STRUCTURE
OF MOLECULAR
IODINE IN SATURATED
April 1978
FLUORESCENCE
E. DOPEL, D. KUHLKE and W. DIETEL Friedrich-Schiller-Universitiit
Jena, Sektion Physik, DDR-69 Jena
Received 3 January 1978
The hyperfine structure of the R(127) 11-5 line (633 nm) of ‘*‘I2 were investigated by means of the saturated fluorescence excited in the resonator of a single mode 3He-20 Ne laser. Fourteen hyperfine components were detected in the tuning range of the laser at pressures from 15 to 240 mtorr.
The hyperfine structure (hfs) of the 12712-molecule (R( 127) line in the 1 l-5 band of the B 311(Oz) - X 1 Z(Oi) electronic transition) was first observed by Hanes and Dahlstrom by means of Doppler-free saturated absorption spectroscopy [l] . They found small narrow peaks from the saturated intracavity absorption of this hfs-components in the output of a single mode 3 He-20Ne laser with a wavelength of 633 mn. We report here for the first time the observation of these hfs-components by means of saturated fluorescence. This method was suggested by Basov and Letokhov [2] and verificated experimentally by Freed and Javan [3] on C02-molecules and by Sorem and Schawlow [4] on 12-molecules in the 5 14 nm region. The technique of the saturated fluorescence is also a method of Doppler-free spectroscopy. In difference to the saturated absorption spectroscopy where the absorption saturated by the counterrunning waves of a monochromatic field is measured in this method the saturated population of a level excited by a monochromatic standing wave field is measured. When tuning the field frequency to the centre frequency of the transition a relative minimum appears in the fluorescence from this excited level. The halfwidth of this dip is determined by the natural width of the exciting transition, Compared with the saturated absorption spectroscopy the method of saturated fluorescence offers several advantages. Since the fluorescence of the molecule is detected the strong laser background is eliminated. Therefore with this method a sensitive 62
saturation spectroscopy of dilute gases of a very low absorption can be performed [3-61. Further this technique is very suitable for the investigation of molecular transitions. Relaxation processes between the vibrational-rotational sublevels of the excited electronic state due to the collisions and velocity changes of the molecules caused also by such collisions diminish the contrast of the absorption dips in the case of saturated absorption spectroscopy [lo] . In the case of saturated fluorescence spectroscopy the contrast is decreased only due to the increasing saturation intensity since the fluorescence from all the vibrational-rotational sublevels of the electronic state is detected [S, 71. In a theoretical treatment the influence of these processes and of the spatially inhomogeneous population distribution of the molecules due to the standing wave pattern on contrast and shape of the fluorescence dips was investigated [7]. In tig. 1 the experimental setup is shown. The iodine cell is placed inside the resonator of a strong single mode He-Ne laser. Single mode operation was achieved by the method of nonlinear absorption [8]. With a He-Ne laser having an active length of 1.70 m and a Ne-tube as nonlinear absorber a power of 35 mW was achieved in single mode operation with an output mirror of R = 96%. However, in order to obtain a high intracavity power in these experiments mirrors with a reflection coefficient >99% were used. The high single mode power and the continuous changing of the laser frequency over more than 500 MHz was achieved by a considerable contribution of homogeneous of the
Volume 25, number 1
April 1978
OPTICS COMMUNICATIONS
abc
hij
defg
klmn cb)
Fig. 1. Schematic drawing of the experimental setup. PM photomultiplier, PZT piezoelectric transducer, SML single mode laser, PC peltier cooler, L lens, S slit, F filter, SIN-GEN sinus generator, RAMP-GEN ramp generator.
laser gain profile [9] . The fluorescing beam inside the quartz iodine cell was imaged with a lens (L) on a slit (S). For registration with the photomultiplier (PM) the antistokes fluorescence was selected with a 600 run interference filter (F). The photomultiplier has to be shielded carefully against scattered light of the laser beam. The vapour pressure of 12 is controled with a peltier cooler (PC). The frequency of the laser beam was periodically shifted by 4 MHz by applying a 500 Hz sinusoidal ac-voltage (SIN-GEN) to a piezoelectric transducer (PZT) which carries one of the end mirrors. The signal was detected with a phase sensitive amplifier. When the laser frequency was tuned slowly over the line first derivative of the hfs-components was obtained. For this tuning a second PZT-element or the thermal laser drift was used. The frequency position of the single mode was controled by optical heterodyning with a wobbeled single mode laser (SML). Figs. 2a and b show typical recorder traces with the fourteen hyperfine components of 127I2 also reported by Hanes and Dahlstrom in saturated absorption. The hfs-components were recorded in the pressure region from 15 to 240 mtorr with an intracavity laser power of about 1 W/cm2. In this pressure region linewidths from 5 to 10 MHz were obtained. We wish to acknowledge the support of Professor V.S. Letokhov and helpful discussions with Professor G. Hesse.
defg
hij
klmn
Fig. 2. Typical recorder traces. (a) Recorded with the following experimental parameters: tuning velocity 1.5 MHz/s, integration constant Is, iodine pressure 170 mtorr, amplitude of frequency modulation 4 MHz, intracavity laser intensity about 1 W/cm’. (b) Recorded with a tuning velocity of 1 MHz/s and a integration constant of 3 s. The well known spacing of the h-i and i-j components of 22 MHz [l] may serve as a frequency scale.
References [l] G.R. Hanes and C.E. Dahlstrom, Appl. Phys. Lett. 14 (1969) 326. [ 21 N.G. Basov and V.S. Letokhov, Report on URSI Conference “Laser Measurements” Sept. 1968 Warsaw, Poland. [3] C. Freed and A. Javan, Appl. Phys. Lett. 17 (1970) 53. [4] M.S. Sorem and A. Schawlow, Opt. Comm. 5 (1972) 148.
63
Volume 25, number 1
OPTICS COMMUNICATIONS
[S] V.S. Letokhov and B.D. Pavlik, Zh. Eksp. i Tear. Fiz. 64 (1973) 804. [6] K. Shimoda, Appl. Phys. 1 (1973) 77. [ 71 E. Diipel and D. Kiihlke, Czech. Journ. Phys., in press. [8] V.P. Chebotayev, LM. Beterov and V.N. Lisitsyn, IEEE J. Quant. Electr. QE (1973) 788.
64
April 1978
[9] W. Dietel, D. Kiihlke and H. Sander, Opt. and Quant. Electr. 7 (1975) 345. [lo] D. Kiihlke and E. DGpel, Czech. Journ. Phys. B25 (1975) 1084.