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A technique for optical pumping in vacuum ultraviolet
Volume
2,
A TECHN
‘IQUE
November 1970
OPTICS COMMUNICATIONS
number 6
FOR
OPTICAL
PUMPING
IN
VACUUM
ULTRAV
IOLET*
G. MORUZZI Scuola
Normale
Superiore,
Italy
Piss,
and F. STRUMIA Istituto
di Fisica
dell’lJniversit&,
Pisa,
Italy
Received 3 September 1970
A new method for obtaining the orientation of atoms by optical means is proposed. Since it achieves the transfer of angular momentum from light to atoms without the need of a linear polarizer and a h/4 plate. it allows the extension of the optical pumping technique into the vacuum ultraviolet spectral region. The efficiency of this method has been studied in the case of “‘Hg.
In an optical pumping experiment the orientation of the atoms is achieved by absorption of angular momentum from light. Thus, the pumping light which is absorbed must be circularly polarized. Usually this is obtained by means of a linear polarizer followed by a X/4 plate. This method has the disadvantage that, since the quality of the available polarizers and plates is qyite unsatisfactory in the far UV (below 2500 A), one can only choose a large energy loss or a poor circular polarization, when working in this spectral region. We avoided this difficulty observing that it is not necessary to polarize the whole excitation beam: as a matter of fact, only the absorbed excitation light needs to be circularly polarized. Making use of the isotopic shift, hyperfine structure and Zeeman effect it is possible to get an excitation light beam, which is unpolarized as a whole, but whose circularly polarized components are shifted in frequency from each other, and only one of them can be absorbed by the atoms to be optically oriented. Let us consider the case of mercury. The ground state is a 1~0, and the even isotopes have I = 0, whereas the two stable odd isotopes have I = + (lggHg) and I = + (201Hg) so that their nuclear orientation can be obtained by means of optical pumping. The hyperfine structure of both_ resonance lines of mercury [lso - lPl (1850A) and lso - 3pl *
Work supported by Gruppo Nazionnle di Strutturn della Materin del CNR, Sezione di Pisa.
(2537 A)] is given in fig. 1. Both the isotope shift and the hyperfine structure (hfs) of these lines are greater than their Doppler width.
2 t 1 t
-I5
-10
-5
0
5
Gliz
Fig. 1. Hyperfine structure of the resonance lines of Hg. The positions of the U components as functions of the magnetic field intensity are lgR Hg in the vlen case of the 2637 A line. andfor OOHgfor in the case of the 1850 A lint.
279
Volume 2. number 6
OPTICS COMMUNICATIONS
If a lamp, filled with an even isotope, is placed in a magnetic field HO, its light emitted in the field direction consists of the two u components of the normal Zeeman multiplet. These components are circularly polarized in opposite directions. The splitting between the a+ and the o- lines increases linearly with the magnetic field intensity. As is shown in fig. 1 the u components reach the positions of the hfs components of the odd isotopes at different values of the field Ho. For example, the c+ component of a lg8Hg lamp can excite only the F =+ line of lggHg at a field of about 3.4 kG, if the 2537 A resonance line is used. With the same resonance line, the (T- component emitted by the same lamp excites the F = 3 line of lggHg at a field of about 7.3 kG. Thus, if we are interested in the optical pumping of 199Hg, we only need to concentrate the light emitted by our lamp at 3.4 kG on the cell containing l99Hg with a lens. Only the or+ Zeeman component is absorbed and pumps the mercury vapour, while the a- is off resonance and does not perturb the orientation process. Analogous results can be obtained with the 1849 A resonance line. This technique is quite general and can be applied to other elements provided that: i) there are several isotopes available ii) the hyperfine splitting of the resonance line is greater than its Doppler width iii) the isotope shift is great enough to permit a different absorption of the o+ and u‘- components.
November
1970
With this method it is possible to extend the optical pumping to elements whose resonance lines are in the vacuum ultraviolet. The possibility to achieve the nuclear orientation of the odd isotopes of xenon and krypton? whose resonance lines are at 1470 and f296 A for Xe, 1236 and 1165 A for Kr, is of particular interest, since very long relaxation times are to be expected [l]. We have tested the efficiency of our method against the usual optical pumping technique in the case of the 2537 A line mercury. The experimental apparatus is shown in fig. 2. The pumping lamp is placed between the poles of a small electromagnet, which has been modified in order to increase the light output in the field direction. A useful solid angle of about 1 - 1.5 sr was achieved. The nuclear orientation of the Hg vapour was detected by monitoring its Faraday rotation [2]. In order to observe the optical pumping with the customary technique the magnet was taken off, and the light emitted by a l99Hg lamp was circularly polarized by means of a good polarizer (Polacoat PL 40) and a X/4 plate consisting of two plates of crystalline quartz each of which is about 0.9 mm thick [3]. Good signals were obtained with both methods. However, there are two points in favour of the use the lamp inside the magnetic field: i) more light is available for the optical pumping process and ii) the experimental apparatus is more stable because the adjusting of the h/4 plate, and there-
Helmholtz
Fig. 2. Experimental apparatus for the optical pumping of 199Hg. A 202Hg lamp, providing non resonant light for the Faraday rotation detection, is used for the monitoring beam. Since the isotopic purity was rather poor, ,a lggHg vapour filter absorbs any unwanted line due to the other Hg isotope. Another filter transmits only the 2537 A line and consists of a water solution of NiS04 and CoSO4 inside a quartz cell [4].
280
Volume
2, number
6
November 1970
OPTICS COMMUNICATIONS
7~125
Fig. 3. The absorption cell. fore the circular polarization of the excitation light, depends critically on temperature and tilting. It is worth noting that the non-resonant component of the pumping beam causes a light-shift in the states of the oriented atoms. This effect is observable in the rf transitions and must be considered in experiments of this kind. However, since the relative positions and intensities of the Zeeman lines are known, the light-shift can easily be computed. In our experience, the Hg vapour to be pumped was placed inside a cylindrical quartz cell, whose windows are optically polished. Our cell is shown in fig. 3. The vapour density is controlled by regulating the temperature of the side arm. The presence of this side arm introduces
S
Fig. 4. Ex erimental results for the nuclear relaxation of 19%Hg. a) cell “apen : bl cell “closed”. a further cause of relaxation. We were able to reduce substantially this drawback by introducing a small quartz or silicone coated glass ball into the cell. After the bottom of the side arm has been cooled down to the temperature which corresponds to the required vapour pressure, one lets the ball roll onto the hole through which the cell and tube communicate. Thus the loss of oriented atoms into the tube is strongly decreased. This corresponds to an increase of the relaxation time, as is shown in fig. 4.
REFERENCES [l] F.J.Adrian, Phys. Rev. 138A (1965) 403. [2] F.Strumia, Nuovo Cimento 44 (1966) 387. [3] P. Violino, Rev. Opt. 44 (1965) 109. [4] M.Kasha, J. Opt. Sot. Am. 3R (1948) 929.
281
2,
A TECHN
‘IQUE
November 1970
OPTICS COMMUNICATIONS
number 6
FOR
OPTICAL
PUMPING
IN
VACUUM
ULTRAV
IOLET*
G. MORUZZI Scuola
Normale
Superiore,
Italy
Piss,
and F. STRUMIA Istituto
di Fisica
dell’lJniversit&,
Pisa,
Italy
Received 3 September 1970
A new method for obtaining the orientation of atoms by optical means is proposed. Since it achieves the transfer of angular momentum from light to atoms without the need of a linear polarizer and a h/4 plate. it allows the extension of the optical pumping technique into the vacuum ultraviolet spectral region. The efficiency of this method has been studied in the case of “‘Hg.
In an optical pumping experiment the orientation of the atoms is achieved by absorption of angular momentum from light. Thus, the pumping light which is absorbed must be circularly polarized. Usually this is obtained by means of a linear polarizer followed by a X/4 plate. This method has the disadvantage that, since the quality of the available polarizers and plates is qyite unsatisfactory in the far UV (below 2500 A), one can only choose a large energy loss or a poor circular polarization, when working in this spectral region. We avoided this difficulty observing that it is not necessary to polarize the whole excitation beam: as a matter of fact, only the absorbed excitation light needs to be circularly polarized. Making use of the isotopic shift, hyperfine structure and Zeeman effect it is possible to get an excitation light beam, which is unpolarized as a whole, but whose circularly polarized components are shifted in frequency from each other, and only one of them can be absorbed by the atoms to be optically oriented. Let us consider the case of mercury. The ground state is a 1~0, and the even isotopes have I = 0, whereas the two stable odd isotopes have I = + (lggHg) and I = + (201Hg) so that their nuclear orientation can be obtained by means of optical pumping. The hyperfine structure of both_ resonance lines of mercury [lso - lPl (1850A) and lso - 3pl *
Work supported by Gruppo Nazionnle di Strutturn della Materin del CNR, Sezione di Pisa.
(2537 A)] is given in fig. 1. Both the isotope shift and the hyperfine structure (hfs) of these lines are greater than their Doppler width.
2 t 1 t
-I5
-10
-5
0
5
Gliz
Fig. 1. Hyperfine structure of the resonance lines of Hg. The positions of the U components as functions of the magnetic field intensity are lgR Hg in the vlen case of the 2637 A line. andfor OOHgfor in the case of the 1850 A lint.
279
Volume 2. number 6
OPTICS COMMUNICATIONS
If a lamp, filled with an even isotope, is placed in a magnetic field HO, its light emitted in the field direction consists of the two u components of the normal Zeeman multiplet. These components are circularly polarized in opposite directions. The splitting between the a+ and the o- lines increases linearly with the magnetic field intensity. As is shown in fig. 1 the u components reach the positions of the hfs components of the odd isotopes at different values of the field Ho. For example, the c+ component of a lg8Hg lamp can excite only the F =+ line of lggHg at a field of about 3.4 kG, if the 2537 A resonance line is used. With the same resonance line, the (T- component emitted by the same lamp excites the F = 3 line of lggHg at a field of about 7.3 kG. Thus, if we are interested in the optical pumping of 199Hg, we only need to concentrate the light emitted by our lamp at 3.4 kG on the cell containing l99Hg with a lens. Only the or+ Zeeman component is absorbed and pumps the mercury vapour, while the a- is off resonance and does not perturb the orientation process. Analogous results can be obtained with the 1849 A resonance line. This technique is quite general and can be applied to other elements provided that: i) there are several isotopes available ii) the hyperfine splitting of the resonance line is greater than its Doppler width iii) the isotope shift is great enough to permit a different absorption of the o+ and u‘- components.
November
1970
With this method it is possible to extend the optical pumping to elements whose resonance lines are in the vacuum ultraviolet. The possibility to achieve the nuclear orientation of the odd isotopes of xenon and krypton? whose resonance lines are at 1470 and f296 A for Xe, 1236 and 1165 A for Kr, is of particular interest, since very long relaxation times are to be expected [l]. We have tested the efficiency of our method against the usual optical pumping technique in the case of the 2537 A line mercury. The experimental apparatus is shown in fig. 2. The pumping lamp is placed between the poles of a small electromagnet, which has been modified in order to increase the light output in the field direction. A useful solid angle of about 1 - 1.5 sr was achieved. The nuclear orientation of the Hg vapour was detected by monitoring its Faraday rotation [2]. In order to observe the optical pumping with the customary technique the magnet was taken off, and the light emitted by a l99Hg lamp was circularly polarized by means of a good polarizer (Polacoat PL 40) and a X/4 plate consisting of two plates of crystalline quartz each of which is about 0.9 mm thick [3]. Good signals were obtained with both methods. However, there are two points in favour of the use the lamp inside the magnetic field: i) more light is available for the optical pumping process and ii) the experimental apparatus is more stable because the adjusting of the h/4 plate, and there-
Helmholtz
Fig. 2. Experimental apparatus for the optical pumping of 199Hg. A 202Hg lamp, providing non resonant light for the Faraday rotation detection, is used for the monitoring beam. Since the isotopic purity was rather poor, ,a lggHg vapour filter absorbs any unwanted line due to the other Hg isotope. Another filter transmits only the 2537 A line and consists of a water solution of NiS04 and CoSO4 inside a quartz cell [4].
280
Volume
2, number
6
November 1970
OPTICS COMMUNICATIONS
7~125
Fig. 3. The absorption cell. fore the circular polarization of the excitation light, depends critically on temperature and tilting. It is worth noting that the non-resonant component of the pumping beam causes a light-shift in the states of the oriented atoms. This effect is observable in the rf transitions and must be considered in experiments of this kind. However, since the relative positions and intensities of the Zeeman lines are known, the light-shift can easily be computed. In our experience, the Hg vapour to be pumped was placed inside a cylindrical quartz cell, whose windows are optically polished. Our cell is shown in fig. 3. The vapour density is controlled by regulating the temperature of the side arm. The presence of this side arm introduces
S
Fig. 4. Ex erimental results for the nuclear relaxation of 19%Hg. a) cell “apen : bl cell “closed”. a further cause of relaxation. We were able to reduce substantially this drawback by introducing a small quartz or silicone coated glass ball into the cell. After the bottom of the side arm has been cooled down to the temperature which corresponds to the required vapour pressure, one lets the ball roll onto the hole through which the cell and tube communicate. Thus the loss of oriented atoms into the tube is strongly decreased. This corresponds to an increase of the relaxation time, as is shown in fig. 4.
REFERENCES [l] F.J.Adrian, Phys. Rev. 138A (1965) 403. [2] F.Strumia, Nuovo Cimento 44 (1966) 387. [3] P. Violino, Rev. Opt. 44 (1965) 109. [4] M.Kasha, J. Opt. Sot. Am. 3R (1948) 929.
281