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Observation of stimulated emission from a simple supersonic jet hollow cathode source
Volume 143, number 6,7
22 January 1990
PHYSICS LETTERS A
OBSERVATION OF STIMULATED EMISSION FROM A SIMPLE SUPERSONIC JET HOLLOW CATHODE SOURCE Peter ERMAN and Peter LINDBLOM Physics Department I, Royal Institute of Technology,
S-100 44 Stockholm,
Sweden
Received 2 October 1989; revised manuscript received 21 November 1989; accepted for publication 21 November 1989 Communicated by B. Fricke
In studies of emission from argon excited in supersonic as well as the static mode in a hollow cathode source, it is observed that some of the Ar II lines appear with subnatural widths and show an intensity amplification indicating the presence of stimulated emission at this kind of incoherent excitations without a resonator.
Knowledge of radiative lifetimes of a manifold of resonance levels in ions of inert gases is of crucial importance for understanding the generation mechanism of ion lasers. Accordingly a large number of papers have been devoted to experimental and theoretical studies of lifetimes of low lying resonance levels, for instance in Ar II. However, since the resonance emission occurs in the VUV region and the lifetimes of the lowest levels (such as A in fig. 2) are frequently in the subnanosecond range, direct measurements of the decay curves are difficult. Therefore attempts have been made to measure for instance the lifetimes ~ of the Ar II states 4s 2P and 3d 2D from the natural linewidths oftransitions from upper states B terminating on these levels. Since TB>> TA the natural linewidths are almost solely determined by the width of the short-lived lower state. Estimates of radiative lifetimes in this way from linewidths could in principle give more accurate resuits in the subnanosecond range than “direct” (time resolved) methods, in particular so if the latter techniques involve errors such as cascade feeding or resonance trapping. Thus, for instance, determinations of Ar II t(4s 2P) from the natural linewidths of the 4p 2P°~~4s 2P transitions [1] yield a result of 0.32— 0.03 ns which is in good agreement with theory [2,3] in contrast to a “direct” measurement using beam foil spectroscopy which gives 0.91 ±0.06ns [4}. Since we have recently developed a new kind of scanning monochromator system with supermillion
resolution [5] and a specially designed supersonic hollow cathode discharge source for sub-Doppler spectroscopy [6], we decided to perform careful linewidth measurements of certain Ar II transitions terminating on short-lived low-lying resonance 1evels. During the experiment we made the remarkable observation that some of these lines occur with considerably smaller widths than the expected natural ones when the light source was operated in the supersonic (cold) mode. The new kind of scanning monochromator of multi Echelle grating (MEGA) type used in the present investigations, has a limiting resolving power of 2.4x 106 at 5000 A and is described in detail elsewhere [5]. The monochromator is equipped with a commercial intensified multichannel detector (IMD, Hamamatsu C2808) which allows a simultaneous registration of 5 12 different spectral positions in any preselected spectral region. A lens images the focal plane of the MEGA spectrometer on the photocathode of the IMD-tube with a magnification of about nine implying that the detector covers about 300 mA with a conversion of 0.6 mA/channel at 5000 A. The exact conversion was calibrated using the hfs-structure of the Hg546 1 and Hg4358 lines from an EDL source. The IMD-tube was scanned at different integration times through computer control of the clock pulse frequency and the recording procedure. As the voltage of the image intensifier tube in the IMD can be varied as well, a large dynamic range can be coy-
0375-960l/90/$ 03.50 © Elsevier Science Publishers B.V. (North-Holland)
297
Volume 143, number 6,7
PHYSICS LETTERS A
ered. The amplification at different voltages and integration times was calibrated to allow relative intensity recordings in large intensity ranges. An electronic shutter was used to allow a “dark” scan after each exposure in order to let the tube recover from memory effects at high intensity levels. Using a tungsten filament lamp the response of each individual channel was calibrated giving a variation of individual channels of <0.5%. Thus the described system makes possible very accurate line profile recordings at high spectral resolution. A simplified drawing of the applied supersonic hollow cathode source (described in more detail in ref. [6]) is shown in fig. I. Argon gas at 15 bar pressure is introduced through a glass tube whose front is shaped into a nozzle. The jet is excited by a discharge between a 20 mm diameter graphite hollow cathode and a ring anode. At normal operation with a 80 i.im nozzle and 1—15 bar stagnation pressure, the pressure of the jet is about 0. 1—0.4 mbar and the discharge current is about I mA. The discharge is concentrated to a cold region a few millimetres from the nozzle and light from this region is selected and imaged by a lens and an aperture onto the entrance slit of the MEGA spectrometer. The spectrometer thus receives the light emitted from the central region in the direction of the beam. The lamp can also be operated in ordinary (static) mode with a continuous flow through of gas from a separate inlet. Measurements of the Doppler spread in a number of atomic transitions with vanishingly small natural widths show a translation temperature of 1 5—18 K when the source is operated in the jet mode and about 320 K in the static mode. The strong Ar II line 4348 A (4p 4D~, 2 to 4P 4s 512, fig. 2) was used as reference line for both the static and the supersonic operation modes. From the calculated lifetimes [2] of the upper and lower
NOZZLE
LENS
SLIT
GRAPHITE CATHODE _____________
Fig. I. Supersonic jet hollow cathode source for sub-Doppler spectroscopy used in the present investigation,
298
22 January 1990
4D
~P°
8.6
B~ ~
___
6 6 ns
A~
~ 455 ns
0.32 ns
Ar~ ~ G.S.
Fig. 2. Partial simplified diagram of some low lying levels in Ar~ investigated in the present work. At non-coherent electron excitation from the Ar ground state in a low power hollow cathode discharge (fig. 1), emission lines from levels B to levels A that have lifetimes TB>>tA, prove to appear with subnatural widths. They are also considerably amplified when the source is operated in the supersonic mode.
states (6.6 and 455 ns, respectively) we estimate the natural width of this transition to 24 MHz thus forming a negligible contribution to the measured total width (about 930 MHz) in the supersonic mode. In addition the transition involved cannot be population inverted. Thus the measured profile of this quartet transition is used as a standard “instrumental” profile for the whole system of source, spectrometer and detector. It has been used in the studies ofabout fifteen low lying doublet and quartet to doublet transitions involving several hundred recordings at different conditions. A typical example can be2P seen in fig. 3 showing the transition 4p ~ to 4s 31., (4545 A) whose expected profile is indicated by the dashed line which is calculated as the convolution of the instrumental profile (dotted) with a Lorentzian distribution with a natural width of 5002PMHz. This width is estimated from 2P~,the values T( 4s 312) = 0.32 ns [1—3] and r(4p 2)= 8.6 ns, the latter value being the average of some ten different measurements (cf. ref. [2]). Indeed in the supersonic mode we observe a line (solid) with a considerably narrower profile than the expected one. In fact the ac-
Volume 143, number 6,7
PHYSICS LETTERS A
22 January 1990
MEASURED PROFILES
OF Ar~ ~ 4545 ~
7
j”INSTRUMENTAL~+EXPECTED
/ / I
Z
/
.~
~.
\‘~
\
INSTRUMENTAL
/ !
NATURAL
1/
1
/ 7’..!
.
ACTU~LYMEASURED ..
1000 tvfliz
CHANNEL NUMBER Fig. 3. Dotted curve: measured “instrumental profile” including the excitation source, spectrometer and detector at 4545 A (FWHM 6.1 Dashed curve: “expected” profile (FWHM 9.6 mA) for the transition Ar~4545 A as calculated from the instrumental profile by convolution with a Lorentzian of the natural width (500 MHz). Solid curve: The actually measured profile ofAr~4545 A (FWHM 5.0 mA). The width is considerable smaller than the expected natural width due to stimulated emission (see text).
mA).
tually measured line is even narrower than the instrumental one. For more details about the recording conditions see the figure caption. Note that the discharge current is only 0.3 mA. We also observe subnatural widths of a number of other Ar II lines terminating on the 4s 2P and 3d 2D levels, for instance the 4880, 5145 and 4765 A lines (see fig. 2). Since all these transitions occur between levels where the inversion condition TB(upper)>> ‘rA(lower) is fulfilled, we expect that a considerable fraction of the light output from our hollow cathode source originates from stimulated emission when the source operates in a supersonic mode. The fact that the observed linewidths are even narrower than the instrumental profile shows that the stimulated emission originates predominantly from the cold (15 K) gas component. The instrumental profile (4348 A quartet line) contains no stimulated component and is therefore widened by the emission from warmer regions in the jet. It is not a coincidence that several of our studied B—A transitions are well known lines emitted from an argon ion laser, To get an independent verification of the inter-
pretation of the subnatural widths as due to stimulated emission, we compared a number of relative line intensities in supersonic and static modes. Fig. 4 illustrates the amplification of some of the Ar II lines in the supersonic mode. To be comparable the intensities have been normalised to the intensity of the 4348 A quartet line which can only be emitted spontaneously. All the lines show an intensity enhancement in the supersonic mode, with an amplification by up to 3.4 for the 4545 A line. Special attention has to be paid to radiative trapping. The reason for this is that all the studied doublet transitions terminate on the short lived 4s 2P~ levels (fig. 2) which subsequently decay through strongly trapped resonance transitions to the Ar II ground state. High populations of the 4s 2p0 levels are thus built up by trapping which gives rise to “double trapping” [7,8] ofthe doublet lines. For the 4s 4P levels no such decay channels exist and “double trapping” of quartet transitions, such as the 4348 A line, is therefore expected to be very small. In view of the close connection between trapping and self-reversal [7,8], we looked for reversed line shapes at 299
Volume 143, number 6,7
2L4545
II
STATIC
Fig. 5 shows a series of recordings of the doublet line 4545 A at varying discharge currents. At a current of only 1.2 mA the reversal is about 30%. The quartet lines, such as 4348 A. show little or no trapping. In the supersonic mode trapping reduces the line intensities but will not reveal itself through deformed or reversed line profiles. As it has been shown
SUPERSON[(
—
2~4880
z
X5145
r
1x3.4
3
II H
II
~4765 I
I\\
/
H
x2.2
I
x 1.4 Ii
II
x23
thantrapping that for the quartets, effects arethe much observed larger intensity for the doublets ampli-
/~ ~
~
fication due to stimulated emission in supersonic mode is only a loweratlimit. It was also found static conditions, that all the lines observed with subnatural widths in supersonic
_______________ ____________________ _____________________
_____________________
static mode discharge current is increased.
when the
RELATIVE INTENSITWS OF SOME TRANSITIONS IN AC WITH SUBNATURAL WmTHS
>. 1-
22 January 1990
PHYSICS LETTERS A
CHANNEL NUMBER
Fig. 4. Relative intensities of some transitions appearing with subnatural widths measured with the source operated in static and in supersonic modes. The intensities are normalised to the “normally behaving” quartet transition 4348 A. The considerable intensity amplifications form an independent confirmation of the laser action of the source in supersonic mode.
mode showed an increase in intensity with increasing discharge current that was much higher than the
corresponding increase for the untrapped quartet line (4348 A). The increase is particularly pronounced for the 5145 A line. This was astonishing as this line at the same time showed reversal at high currents. Investigation of line profiles at low currents also showed a structure with a narrow profile due to stimulated emission superimposed on a broader profile from spontaneous emission. These observations sup-
static conditions in a 50 mm long hollow cathode at discharge currents > 1 mA. All the doublet lines observed with subnatural widths in supersonic mode are trapped, as revealed by their reversed profiles
MEASURED PROFILES OF A.R~X 4545 A AT
.-.,
~.--‘
7
~
I / CURRENTS IN STATIC / / OPERATION / / VARIOUS DISCHARGE
z
.7
\\. I
I
1.2mA~f
I.’
\
I\
I
~
I? II
I / /
/
0.9mA~.
.—0.3mA
,‘
I
I
~
I
I
;‘
1000~
II,
.-
•
::~.....•.
iOmA
•
I
~/.
I
~
:~
CHANNEL NUMBER Fig. 5. The Ar~4545 transition measured in a static 50 mm hollow cathode discharge at three different currents. Dotted curve: 0.3 mA, dashed curve: 0.9 mA and solid curve: 1.2 mA. The reversal at 1.2 mA isabout 30%. Note the increasing shift towards longer wavelengths at increased trapping due to inelastic collision processes.
300
Volume 143, number 6,7
PHYSICS LETTERS A
port the conclusion that stimulated emission also occurs in the static mode. The profiles of these lines in static mode are thus composed of a blend of stimulated and spontaneous emission as well as trapping. Thus, in a summary, we propose that the following excitation mechanism takes place in our supersonic hollow cathode lamp (fig. 1). Free electrons in the continuous discharge excite the Ar atoms in the jet to state B, either directly or via intermediate states or cascading states. Thus, a considerable population of the state B is created which subsequently decays to state A. For a number of combinations of states (A, B) TB>> TA and an inverted population is built up forming the condition for stimulated emission from B and the creation of a coherent radiation field in the direction of the jet. For atoms moving in other directions the Doppler width is almost three times larger than the natural width and the narrowing effect would be hard to detect. However, the emission from the majority of these atoms will escape the acceptance angle of the spectrometer. Since the excitation takes place along a small section of the jet, only a modest amplification will be achieved in the absence of any resonance cavity. A modified version ofthe present lamp is therefore under construction where the nozzle will be shaped as a slit thus giving a jet with a considerably longer cold excitation region. As this cold region extends in the direction of the slit a resonator can easily be added to the system. In this way laser action should be obtamed [9]. In principle a series of such slits can be aligned into a long laser cavity. As the laser action predominantly takes place in the cold (1 5 K) region of the jets, this laser has only one longitudinal mode up to cavity lengths of about 0.5 m. In this context the substantial difference between the present source and ordinary hollow cathode lasers should be emphasised (cf. refs. [10,11]). In the latter sources high
22 January 1990
current (5—10 A) discharges are created in hot gas plasmas at fairly high pressures in long (30—50 cm) hollow cathodes. The inversion is obtained in a chain of collision processes resulting in amplification in the usual manner using resonators. In the present supersonic source, the amplification is obtained in a small (<1 cm) part of a 15 K gas jet without any resonator in a very low power discharge (<1 mA) at pressures of the order of 0. 1 mbar. Finally it should be mentioned that the possibilities to use the present source as an ion source will also be investigated. Since the energies of the ions are much lower (<0.1 eV) and the current densities higher than in conventional ion sources, it could be most useful in various kinds of experiments such as photon—ion excitations or determinations oflong radiative lifetimes in ions using the time-of-flight technique.
References [1] F.A. [2] [3] [4] [5] [6] [7]
Korolyev, V.V. Lebedeva, A.E. Novik and Al. Odintsov, Opt. Spektrosk. 33 (1972) 435. A.V. Loginov and P.F. Gruzdev, Opt. Spektrosk. 44 (1978) G.F. Koster, H. Statz and C.L. Tang, J. Appl. Phys. 39 (1968) 4045. A.E. Livingston, D.J.G. Irwing and E.H. Pinnington, J. Opt. Soc. 62 (1972) 1303. 0. Gustafsson and P. Lindblom, AppI. Opt. 27 (1988) 147. P. Erman, 0. Gustafsson and P. Lindblom, Phys. Scr. 38 (1988) 789. P. Erman, 0. Gustafsson and P. Lindblom, Phys. Scr. 37 (1988) 42.
[8] P. Erman and S. Huldt, Phys. Scr. 17 (1978) 473. [9] P. Erman and P. Lindblom, Patent pending. 110] M. Jánossy, K. Rózsa and L. Csillag, Europhys. News 13 (1982) 9. [11] M. Jánossy, K. ROzsa, P. Apai and L. Csillag, Opt. Commun. 49 (1984) 178.
301
22 January 1990
PHYSICS LETTERS A
OBSERVATION OF STIMULATED EMISSION FROM A SIMPLE SUPERSONIC JET HOLLOW CATHODE SOURCE Peter ERMAN and Peter LINDBLOM Physics Department I, Royal Institute of Technology,
S-100 44 Stockholm,
Sweden
Received 2 October 1989; revised manuscript received 21 November 1989; accepted for publication 21 November 1989 Communicated by B. Fricke
In studies of emission from argon excited in supersonic as well as the static mode in a hollow cathode source, it is observed that some of the Ar II lines appear with subnatural widths and show an intensity amplification indicating the presence of stimulated emission at this kind of incoherent excitations without a resonator.
Knowledge of radiative lifetimes of a manifold of resonance levels in ions of inert gases is of crucial importance for understanding the generation mechanism of ion lasers. Accordingly a large number of papers have been devoted to experimental and theoretical studies of lifetimes of low lying resonance levels, for instance in Ar II. However, since the resonance emission occurs in the VUV region and the lifetimes of the lowest levels (such as A in fig. 2) are frequently in the subnanosecond range, direct measurements of the decay curves are difficult. Therefore attempts have been made to measure for instance the lifetimes ~ of the Ar II states 4s 2P and 3d 2D from the natural linewidths oftransitions from upper states B terminating on these levels. Since TB>> TA the natural linewidths are almost solely determined by the width of the short-lived lower state. Estimates of radiative lifetimes in this way from linewidths could in principle give more accurate resuits in the subnanosecond range than “direct” (time resolved) methods, in particular so if the latter techniques involve errors such as cascade feeding or resonance trapping. Thus, for instance, determinations of Ar II t(4s 2P) from the natural linewidths of the 4p 2P°~~4s 2P transitions [1] yield a result of 0.32— 0.03 ns which is in good agreement with theory [2,3] in contrast to a “direct” measurement using beam foil spectroscopy which gives 0.91 ±0.06ns [4}. Since we have recently developed a new kind of scanning monochromator system with supermillion
resolution [5] and a specially designed supersonic hollow cathode discharge source for sub-Doppler spectroscopy [6], we decided to perform careful linewidth measurements of certain Ar II transitions terminating on short-lived low-lying resonance 1evels. During the experiment we made the remarkable observation that some of these lines occur with considerably smaller widths than the expected natural ones when the light source was operated in the supersonic (cold) mode. The new kind of scanning monochromator of multi Echelle grating (MEGA) type used in the present investigations, has a limiting resolving power of 2.4x 106 at 5000 A and is described in detail elsewhere [5]. The monochromator is equipped with a commercial intensified multichannel detector (IMD, Hamamatsu C2808) which allows a simultaneous registration of 5 12 different spectral positions in any preselected spectral region. A lens images the focal plane of the MEGA spectrometer on the photocathode of the IMD-tube with a magnification of about nine implying that the detector covers about 300 mA with a conversion of 0.6 mA/channel at 5000 A. The exact conversion was calibrated using the hfs-structure of the Hg546 1 and Hg4358 lines from an EDL source. The IMD-tube was scanned at different integration times through computer control of the clock pulse frequency and the recording procedure. As the voltage of the image intensifier tube in the IMD can be varied as well, a large dynamic range can be coy-
0375-960l/90/$ 03.50 © Elsevier Science Publishers B.V. (North-Holland)
297
Volume 143, number 6,7
PHYSICS LETTERS A
ered. The amplification at different voltages and integration times was calibrated to allow relative intensity recordings in large intensity ranges. An electronic shutter was used to allow a “dark” scan after each exposure in order to let the tube recover from memory effects at high intensity levels. Using a tungsten filament lamp the response of each individual channel was calibrated giving a variation of individual channels of <0.5%. Thus the described system makes possible very accurate line profile recordings at high spectral resolution. A simplified drawing of the applied supersonic hollow cathode source (described in more detail in ref. [6]) is shown in fig. I. Argon gas at 15 bar pressure is introduced through a glass tube whose front is shaped into a nozzle. The jet is excited by a discharge between a 20 mm diameter graphite hollow cathode and a ring anode. At normal operation with a 80 i.im nozzle and 1—15 bar stagnation pressure, the pressure of the jet is about 0. 1—0.4 mbar and the discharge current is about I mA. The discharge is concentrated to a cold region a few millimetres from the nozzle and light from this region is selected and imaged by a lens and an aperture onto the entrance slit of the MEGA spectrometer. The spectrometer thus receives the light emitted from the central region in the direction of the beam. The lamp can also be operated in ordinary (static) mode with a continuous flow through of gas from a separate inlet. Measurements of the Doppler spread in a number of atomic transitions with vanishingly small natural widths show a translation temperature of 1 5—18 K when the source is operated in the jet mode and about 320 K in the static mode. The strong Ar II line 4348 A (4p 4D~, 2 to 4P 4s 512, fig. 2) was used as reference line for both the static and the supersonic operation modes. From the calculated lifetimes [2] of the upper and lower
NOZZLE
LENS
SLIT
GRAPHITE CATHODE _____________
Fig. I. Supersonic jet hollow cathode source for sub-Doppler spectroscopy used in the present investigation,
298
22 January 1990
4D
~P°
8.6
B~ ~
___
6 6 ns
A~
~ 455 ns
0.32 ns
Ar~ ~ G.S.
Fig. 2. Partial simplified diagram of some low lying levels in Ar~ investigated in the present work. At non-coherent electron excitation from the Ar ground state in a low power hollow cathode discharge (fig. 1), emission lines from levels B to levels A that have lifetimes TB>>tA, prove to appear with subnatural widths. They are also considerably amplified when the source is operated in the supersonic mode.
states (6.6 and 455 ns, respectively) we estimate the natural width of this transition to 24 MHz thus forming a negligible contribution to the measured total width (about 930 MHz) in the supersonic mode. In addition the transition involved cannot be population inverted. Thus the measured profile of this quartet transition is used as a standard “instrumental” profile for the whole system of source, spectrometer and detector. It has been used in the studies ofabout fifteen low lying doublet and quartet to doublet transitions involving several hundred recordings at different conditions. A typical example can be2P seen in fig. 3 showing the transition 4p ~ to 4s 31., (4545 A) whose expected profile is indicated by the dashed line which is calculated as the convolution of the instrumental profile (dotted) with a Lorentzian distribution with a natural width of 5002PMHz. This width is estimated from 2P~,the values T( 4s 312) = 0.32 ns [1—3] and r(4p 2)= 8.6 ns, the latter value being the average of some ten different measurements (cf. ref. [2]). Indeed in the supersonic mode we observe a line (solid) with a considerably narrower profile than the expected one. In fact the ac-
Volume 143, number 6,7
PHYSICS LETTERS A
22 January 1990
MEASURED PROFILES
OF Ar~ ~ 4545 ~
7
j”INSTRUMENTAL~+EXPECTED
/ / I
Z
/
.~
~.
\‘~
\
INSTRUMENTAL
/ !
NATURAL
1/
1
/ 7’..!
.
ACTU~LYMEASURED ..
1000 tvfliz
CHANNEL NUMBER Fig. 3. Dotted curve: measured “instrumental profile” including the excitation source, spectrometer and detector at 4545 A (FWHM 6.1 Dashed curve: “expected” profile (FWHM 9.6 mA) for the transition Ar~4545 A as calculated from the instrumental profile by convolution with a Lorentzian of the natural width (500 MHz). Solid curve: The actually measured profile ofAr~4545 A (FWHM 5.0 mA). The width is considerable smaller than the expected natural width due to stimulated emission (see text).
mA).
tually measured line is even narrower than the instrumental one. For more details about the recording conditions see the figure caption. Note that the discharge current is only 0.3 mA. We also observe subnatural widths of a number of other Ar II lines terminating on the 4s 2P and 3d 2D levels, for instance the 4880, 5145 and 4765 A lines (see fig. 2). Since all these transitions occur between levels where the inversion condition TB(upper)>> ‘rA(lower) is fulfilled, we expect that a considerable fraction of the light output from our hollow cathode source originates from stimulated emission when the source operates in a supersonic mode. The fact that the observed linewidths are even narrower than the instrumental profile shows that the stimulated emission originates predominantly from the cold (15 K) gas component. The instrumental profile (4348 A quartet line) contains no stimulated component and is therefore widened by the emission from warmer regions in the jet. It is not a coincidence that several of our studied B—A transitions are well known lines emitted from an argon ion laser, To get an independent verification of the inter-
pretation of the subnatural widths as due to stimulated emission, we compared a number of relative line intensities in supersonic and static modes. Fig. 4 illustrates the amplification of some of the Ar II lines in the supersonic mode. To be comparable the intensities have been normalised to the intensity of the 4348 A quartet line which can only be emitted spontaneously. All the lines show an intensity enhancement in the supersonic mode, with an amplification by up to 3.4 for the 4545 A line. Special attention has to be paid to radiative trapping. The reason for this is that all the studied doublet transitions terminate on the short lived 4s 2P~ levels (fig. 2) which subsequently decay through strongly trapped resonance transitions to the Ar II ground state. High populations of the 4s 2p0 levels are thus built up by trapping which gives rise to “double trapping” [7,8] ofthe doublet lines. For the 4s 4P levels no such decay channels exist and “double trapping” of quartet transitions, such as the 4348 A line, is therefore expected to be very small. In view of the close connection between trapping and self-reversal [7,8], we looked for reversed line shapes at 299
Volume 143, number 6,7
2L4545
II
STATIC
Fig. 5 shows a series of recordings of the doublet line 4545 A at varying discharge currents. At a current of only 1.2 mA the reversal is about 30%. The quartet lines, such as 4348 A. show little or no trapping. In the supersonic mode trapping reduces the line intensities but will not reveal itself through deformed or reversed line profiles. As it has been shown
SUPERSON[(
—
2~4880
z
X5145
r
1x3.4
3
II H
II
~4765 I
I\\
/
H
x2.2
I
x 1.4 Ii
II
x23
thantrapping that for the quartets, effects arethe much observed larger intensity for the doublets ampli-
/~ ~
~
fication due to stimulated emission in supersonic mode is only a loweratlimit. It was also found static conditions, that all the lines observed with subnatural widths in supersonic
_______________ ____________________ _____________________
_____________________
static mode discharge current is increased.
when the
RELATIVE INTENSITWS OF SOME TRANSITIONS IN AC WITH SUBNATURAL WmTHS
>. 1-
22 January 1990
PHYSICS LETTERS A
CHANNEL NUMBER
Fig. 4. Relative intensities of some transitions appearing with subnatural widths measured with the source operated in static and in supersonic modes. The intensities are normalised to the “normally behaving” quartet transition 4348 A. The considerable intensity amplifications form an independent confirmation of the laser action of the source in supersonic mode.
mode showed an increase in intensity with increasing discharge current that was much higher than the
corresponding increase for the untrapped quartet line (4348 A). The increase is particularly pronounced for the 5145 A line. This was astonishing as this line at the same time showed reversal at high currents. Investigation of line profiles at low currents also showed a structure with a narrow profile due to stimulated emission superimposed on a broader profile from spontaneous emission. These observations sup-
static conditions in a 50 mm long hollow cathode at discharge currents > 1 mA. All the doublet lines observed with subnatural widths in supersonic mode are trapped, as revealed by their reversed profiles
MEASURED PROFILES OF A.R~X 4545 A AT
.-.,
~.--‘
7
~
I / CURRENTS IN STATIC / / OPERATION / / VARIOUS DISCHARGE
z
.7
\\. I
I
1.2mA~f
I.’
\
I\
I
~
I? II
I / /
/
0.9mA~.
.—0.3mA
,‘
I
I
~
I
I
;‘
1000~
II,
.-
•
::~.....•.
iOmA
•
I
~/.
I
~
:~
CHANNEL NUMBER Fig. 5. The Ar~4545 transition measured in a static 50 mm hollow cathode discharge at three different currents. Dotted curve: 0.3 mA, dashed curve: 0.9 mA and solid curve: 1.2 mA. The reversal at 1.2 mA isabout 30%. Note the increasing shift towards longer wavelengths at increased trapping due to inelastic collision processes.
300
Volume 143, number 6,7
PHYSICS LETTERS A
port the conclusion that stimulated emission also occurs in the static mode. The profiles of these lines in static mode are thus composed of a blend of stimulated and spontaneous emission as well as trapping. Thus, in a summary, we propose that the following excitation mechanism takes place in our supersonic hollow cathode lamp (fig. 1). Free electrons in the continuous discharge excite the Ar atoms in the jet to state B, either directly or via intermediate states or cascading states. Thus, a considerable population of the state B is created which subsequently decays to state A. For a number of combinations of states (A, B) TB>> TA and an inverted population is built up forming the condition for stimulated emission from B and the creation of a coherent radiation field in the direction of the jet. For atoms moving in other directions the Doppler width is almost three times larger than the natural width and the narrowing effect would be hard to detect. However, the emission from the majority of these atoms will escape the acceptance angle of the spectrometer. Since the excitation takes place along a small section of the jet, only a modest amplification will be achieved in the absence of any resonance cavity. A modified version ofthe present lamp is therefore under construction where the nozzle will be shaped as a slit thus giving a jet with a considerably longer cold excitation region. As this cold region extends in the direction of the slit a resonator can easily be added to the system. In this way laser action should be obtamed [9]. In principle a series of such slits can be aligned into a long laser cavity. As the laser action predominantly takes place in the cold (1 5 K) region of the jets, this laser has only one longitudinal mode up to cavity lengths of about 0.5 m. In this context the substantial difference between the present source and ordinary hollow cathode lasers should be emphasised (cf. refs. [10,11]). In the latter sources high
22 January 1990
current (5—10 A) discharges are created in hot gas plasmas at fairly high pressures in long (30—50 cm) hollow cathodes. The inversion is obtained in a chain of collision processes resulting in amplification in the usual manner using resonators. In the present supersonic source, the amplification is obtained in a small (<1 cm) part of a 15 K gas jet without any resonator in a very low power discharge (<1 mA) at pressures of the order of 0. 1 mbar. Finally it should be mentioned that the possibilities to use the present source as an ion source will also be investigated. Since the energies of the ions are much lower (<0.1 eV) and the current densities higher than in conventional ion sources, it could be most useful in various kinds of experiments such as photon—ion excitations or determinations oflong radiative lifetimes in ions using the time-of-flight technique.
References [1] F.A. [2] [3] [4] [5] [6] [7]
Korolyev, V.V. Lebedeva, A.E. Novik and Al. Odintsov, Opt. Spektrosk. 33 (1972) 435. A.V. Loginov and P.F. Gruzdev, Opt. Spektrosk. 44 (1978) G.F. Koster, H. Statz and C.L. Tang, J. Appl. Phys. 39 (1968) 4045. A.E. Livingston, D.J.G. Irwing and E.H. Pinnington, J. Opt. Soc. 62 (1972) 1303. 0. Gustafsson and P. Lindblom, AppI. Opt. 27 (1988) 147. P. Erman, 0. Gustafsson and P. Lindblom, Phys. Scr. 38 (1988) 789. P. Erman, 0. Gustafsson and P. Lindblom, Phys. Scr. 37 (1988) 42.
[8] P. Erman and S. Huldt, Phys. Scr. 17 (1978) 473. [9] P. Erman and P. Lindblom, Patent pending. 110] M. Jánossy, K. Rózsa and L. Csillag, Europhys. News 13 (1982) 9. [11] M. Jánossy, K. ROzsa, P. Apai and L. Csillag, Opt. Commun. 49 (1984) 178.
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