Population Inversion Mechanisms and Kinetics in Atomic Fluorine Laser H. Mehravaran*
P. Parvin
Physics Department, Karaj Branch, Islamic Azad University, Karaj, Iran,
[email protected]
Physics Department, Amirkabir University of Technology, Tehran, Iran
*
S. Zabani Physics Department, Karaj Branch, Islamic Azad University, Karaj, Iran
In study on laser spectral lines intensities and relation to kinetics and pumping process there are several parameters which we have to perpend. Indeed, laser output is affected, not only by the initial feeding processes, but also by the relaxation and mixing processes, in both upper and lower lasing levels, and the collisions with neutral atoms and electrons reflect on the distribution of intensities among atomic transitions. Here we attention to effect of population inversion because of absorption and stimulated emission cross-section and collision line broadening.
Abstract: We have done study on kinetics in atomic fluorine laser based on population inversion. The absorption and stimulated emission cross-section and also line broadening scrutinize.
Keywords: Kinetics, Atomic fluorine laser, Population inversion mechanisms, absorption cross-section 1
Introduction
In our previous works, six distinguished F* spectral lines such as 6348, 6414, 7128, 7311, 7399 and 755.2 nm were obviously seen 1,2. Here, another specific F* line at 751.5 nm° was also detected because of using monochromator with better resolution in separation of spectral lines. The corresponding line intensities alter nonlinearly with pressure from 2 psi (absolute) up to 5 atm (1 atm=14.7 psi). Kinetics among the spectral lines of each submanifold S, P and D has been contemplated to explain how the line strength varies. The selection rules have been used to identify the allowed and forbidden transitions particularly in the atomic fluorine laser. The correlations between He+ and He2+ species with pumping mechanisms and observing spectral lines have been extensively studied.
2
Experimental Apparatus
We used a home made laser device which had been filled with He and fluorine in 0.2% like the previous work which we had explained [1-2]. The wavelengths and the relative intensities of the atomic fluorine laser were measured with a monochromator (Acton VM-502) having resolution 0.1 nm and a 400-MHz Tektronix 7844 oscilloscope. Because of better resolution in monochromator we could find a new spectral line in infrared quartet submanifold which has been lost in width of the other lines before. A joule meter (Coherent. Filed Master, LM-P10 and LM-P5 LP heads) was used for the absoluteenergy measurement of laser pulse too.
1
3
Table 1: Calculated 17 spectral lines of atomic fluorine laser absorption and stimulated emission cross-section [4] and spontaneous emission rate (Aki) [3]
Results and Discussion
In this special laser which we have 27 allowed spectral lines with the same lower level on some submanifolds we can study and prove on population inversion principal. So in previous work on the atomic fluorine laser from low to high tube pressure we observe 7 spectral lines. Above 2 atm another spectral line (751.5 nm) was seen here that grows up very rapidly up to 5 atm. This line was not previously detected [1], using high resolution spectrometer in this work to resolve two adjacent lines. We explained about some effect of parametric in line intensities and now we focus on two main things in stimulated and absorption cross-sections and the spontaneous emission probability and broadening. We use those because of explanation of the rest principal line intensities and gain competition. The stimulated and absorption cross-sections and the spontaneous emission probability [3] were listed in Table 1 in order to explain the relative intensity of F* laser transitions.
λ
Aki 8
-1
(nm) (10 sec )
gi gk
σ 12
The atomic absorption cross-section at centre wavelength λ12 is associated with the transition between ground state 1 and the upper state 2 given by [4]:
σ12 = σ ref
and wavelength λ ref are 0.1523 × 10-6 m2s-1, 0.1367 × 108 s-1 and 529.177 nm respectively. The notation λ12 , A21, g1, g2 ascertain absorption wavelength, A-factor and degeneracy of states 1 and 2 or gi and gk , respectively. γ e denotes the sum of all spontaneous emission and the inelastic collision rates as well as the twice deactivation elastic collision rates. The value for a collisionally dominated medium is given to be ~ 1 GHz [4]:
(10-12 cm2) (10-12 cm2)
0.25
6 4
10.32
15.48
634.85
0.18
4 4
11.53
11.53
641.37
0.11
2 4
14.39
7.19
696.64
0.11
4 2
4.24
8.49
703.75
0.3
4 4
23.62
23.62
712.79
0.38
2 2
30.70
30.70
731.1
0.39
4 2
16.57
33.14
739.87
0.285
6 6
24.81
24.81
742.57
0.34
4 2
14.90
29.81
748.27
0.056
4 4
4.99
4.99
748.92
0.11
2 2
9.81
9.81
751.49
0.052
2 2
4.67
4.67
755.22
0.078
4 6
10.61
7.07
670.83
0.14
6 4
-
-
760.72
0.07
4 4
6.4
6.4
775.47
0.382
4 6
-
-
780.02
0.21
2 4
-
-
(1)
Where reference of cross-section σ ref , A-factor Aref
σ 21
623.97
4 g 2 λ ref 2 A 21 ( )( ) ( ) γ e g1 λ12 A ref
γe = ∑ A1i + ∑ A2i +∑ k1iinelastic + ∑ kinelastic + 2i i
2k1elastic
i
+ 2kelastic 2
i
i
(2)
In atomic fluorine laser the gain medium is homogeneously broadened by collisional effects at high pressures. The collisional or pressure broadening was typical calculated to be ~ 38 GHz at 5.5 atm. Conversely, the inhomogeneous doppler broadening for 6384.8 nm was determined to be around 1.3 GHz where ∆ υ C / ∆ υ D becomes significantly greater than unity to pronounce the homogeneous broadening is dominant. The natural broadening is near 5.7 MHz which we can pass it up. This broadening emanate from spontaneous emission which we list in table 1. Stimulated emission cross-section σ 21 is equal to g1 σ which we have not calculated for some g 2 12 spectral lines because of they observe in low pressure and obtaining γ e is impossible. In addition the gain competition of atomic fluorine lines is depending on population inversion and the corresponding spontaneous emission rate A21 according to G = ∆ × σ st . For transition having
2
generalize this concept to all other spectral lines. Consequently, each transition in atomic fluorine laser notably nominates the empty lower states to provide the principle of population inversion. Although the populations of the upper and the lower states deplete due to the collisional quenching, however that of lower state decays much more rapidly resulting in the gain growth.
the same lower level, it is at benefit of those possess greater A21. In our experiments [1] the spectral lines of 739.9 nm and 751.5 nm occur with empty lower states because of diminish of 641.4 nm transition. The anomaly happens for wavelength 742.6 nm which is principally a strong emission due to the great A21, however it was not detected. The upper level is the same as adjacent transition 751.5 nm and the lower level is shared with spectral lines of 755.2 nm. We have observed 641.4 and 634.8 nm with lower A21 populate upper levels significantly due to higher absorption cross-sections (Table 1). Moreover, when the lower state of laser transition is occupied during the deactivations process of a specific line within the manifold, the population inversion imparts to that transition with greater transition probability. On the other hand, lines intensities mainly depend on population inversion or empty lower levels and then spontaneous emission probability with participation pumping species. The transition corresponding to 760.7 nm line (3p2D3/2 → 3s2P3/2) has not been seen yet, mainly because of low transition probability (A21≈ 0.07 × 108 sec-1) and the gain competition. The latter arises from the fact that its lower state is shared with another intense transition 775.5 nm.
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References [1]
P. Parvin, H.Mehravaran and B.Jaleh, "Spectral lines of the atomic fluorine laser from 2psi(absolute) to 5.5 atm," Applied Optics, Vol 40, No. 21, pp. 3532-3538, 2001. [2] H. Mehravaran, P. Parvin and D. Dorranian, "Changeover in the molecular and atomic fluorine laser transitions" Applied Optics, Vol. 49, No.15, pp. 2741-2748, 2010. [3] D. R. Lide, Handbook of chemistry and
physics, CRC Press, 84th ed., 2004. [4] O. Axner, J. Gustafsson, N. Omenetto and J. D. Winefordner," Line strengths, Afactors and absorption cross-section for fine structure lines in multiplets and hyperfine structure components in lines in atomic spectrometry – a user's guide," Spectrochimica Acta, B., Vol. 59, pp. 1-39 2004.
Conclusion
The main point is the spectral lines in each laser transitions have a same treatment and we can
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