Na2b3∑+g–3∑+u excimer laser emission in the ir

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Na, b 3t;p’- 3 Z;i EXCIMER LASER EMISSION IN THE ir S.G. DlNEV, LG. KOPRINKOV and I.L. STEFANOV Department of Physics, Soft2 University, 1 I26 Sofia, Bulgaria Received 13 August 1984

We report the first observation of excimer laser action in the near ir region in Naz b 3~$’ = 0) -t x 3~L bound-free transition centered at 830 mn. It was optically excited by a pulsed dye laser. A mechanism of collisional energy transfer is proposed for the population of the Na, upper laser state.

1. Introduction

2. Experimental

Excimer lasers have been thoroughly invehtigated and found a wide practical application due to their large emission cross section, tunability and scalability. Their emission is on discrete wavelengths in the ultraviolet and visible region [ 1] . Optically pumped homonuclear diatomic (dimer) lasers have been developed in recent years. In these systems discrete output lines are obtained because they operate on bound-bound transitions between strongly bound electronic states [2-4] . Bound-bound and bound-free fluorescence in Na2 A 1X:-X 1x: singlet system was studied theoretically and expenmentally in ref. [5 1. Very recently Bahns et al. [6] reported discrete laser line emission and two relatively broad maxima about 804 nm and 818 nm, corresponding to transitions to high lying vibrational states and bound-free transitions of Na, X ?Z+. Laser induced fluorescence in NaK d 311l-a 3YZ8continuum has been investigated both theoretically and experimentally by several authors [7-91. Excimer laser emission in the triplet system 3Z:- 3Zi of Li2 and Na2 has been predicted by Konowalow and Julienne [lOI * In this work we report on what we believe to be the first observation of near ir excimer laser emission in the Na, triplet system b 3Zz + x 3Zz. A novel optical pumping method is employed to overcome the spin selection rules for pumping of the upper laser triplet state.

Sodium metal was enclosed in a 5 1 cm heat pipe oven made of stainless steel. The temperature of the heat pipe was varied from 500 K to 700 K. Ar buffer gas of fixed pressure of 30 Torr was introduced to prevent fogging of the LiF windows. A Rh6G dye laser pumped by N2 laser (Lambda Physik K500 and FL2000) was used for optical pumping. The emission wavelength was measured with an optical spectrum analyzer (B&M OSA 500) with different gratings. The instrumental resolution with a 1200 P mm-l grating was 0.5 A channel-l. Calibration was done by a Ne spectral lamp.

0 030-4018/84/$03.00 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

3. Excimer emission in Na, 3Zi + 3ZZisystem The relevant potential curves of interest of Na, are plotted in fig. 1 using the calculated data of Konowalow, Rosenkranz and Olson [ 1 l] , which are claimed to be among the most accurate theoretical results currently available. Ab initio calculations of Konowalow and Julienne [lo] suggest that the 3xi-32+ system of both Liz and Na, may be used as ar! excirr& laser or tunable’ optical converter in the near ir . In the case of Na,, laser action was predicted between moderately high vibrational levels u’ = 10 of b 3Zi state and the ground x 3Ci state. A rotationless analysis based on quantum mechanical calculations shows that the dominant emission is to the x 3Z: continuum, 199

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4. Pumping mechanism

3’6/2,y2b32s~,2

Fig. 1. Potential energy b “.Zi + x “C$ excimer

curves of the optically laser.

pumped

Naz

although a significant fraction (l/4) of the emission is to the bound levels of x 3 ZZ: (dissociation energy De = 180.2 cm-l [l 11). For example, the transitions from the lowest vibrational level u’ = 0 of b 3E+ have emission coefficients of A = 6.74 X lo7 s-l angd 0.02 X 107 s-l [l I] in the bound-free and bound-bound case, respectively. According to the authors [lo], the peak gain cross section u, = X2hA/8n for u’ = 0 is 18 a2 at h = 830 nm (compared with the peak gain cross section u, = 2.4 A2, measured for the KrF extimer laser [ I] ). Weaker bound-free b 3Ci -+ x ‘EC emission is to be expected from higher vibrational levels on the short-wavelength side. The difficulties in constructing a practical laser system arise from self absorption and availability of a suitable pump for the 3C+ state. Although the lower level of the excimer transit&n is essentially dissociative, population caused by colliding atoms provides free-bound absorption, thus setting a lower limit on the excited state density required in order to achieve net positive gain [ 111. The conditions of detailed balance between stimulated bound-free emission and free-bound absorption has been discussed by Mies and Julienne [ 12 ] . It has been shown that it seems likely that there are accessible conditions of temperature and pressure where free-bound absorption is not a problem over a reasonable tuning range of frequencies. 200

The most significant problem to be solved experimentally is how to pump the relatively short lived (13-15 ns) [ll] upper triplet b 3Zi state. It was demonstrated by several workers in the case of NaK that spin selection rules are relaxed due to perturbation between D ‘Il, and d 3111 states, leading to d 3111-a 3 Z+ fluorescence [7,8] . Here we employ a different approach based on energy transfer between different species. In a previous work [ 131 we have proposed and used collisional energy transfer from excited atomic levels in order to achieve inversion population on a number of bound-bound transitions in the singlet Na, A ‘xi -+ X ‘E’ system. Making use of this mechanism we were a%le to transfer population effectively also in the triplet system under consideration, see fig. 1. We pump with two photons 4D state of atomic sodium. Collisions between excited Na(4D) and ground state atoms form sodium dimer, following the reactions: Na*(4D) + Na(3S) -+ Na;[‘Z(3S

t 4D), 3Z(3S + 4D), ...I.

(1)

Na2[32(3S

t 4D), ...I cascading+ Naz(b 3Ei). (2) transitions In contrast to the bound-bound singlet emission pumped via both 4D and 5s sodium states [ 131, in this experiment by tuning the pump laser wavelength from 570 nm to 604 nm, we were able to excite the excimer emission only through the stronger 3S-4D resonance. Molecular X ’ Ei + A ‘2: absorption and weak predissociation to triplet states [7,8 ] seems to be not effective enough. In principle, colliding 3P atoms can also provide population of the upper triplet states. However, the pump beam is strongly depleted in the first few centimeters in the vapor column with a reasonable density. Therefore, in the experiment of Allegrini et al. [ 141, where Na(3P) state was pumped and singlet Na, and NaK fluorescence was measured, the cw dye laser beam was slightly focussed just behind the input window and the fluorescence light was observed from the same side of the cell.

5. Characteristics of the excimer emission With the dye laser wavelength tuned to the 3S-4D

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h30lml

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Fig. 2. (a) OSA plot of the Na2 excimer 830 nm and satellite bound-bound 840.24 nm emission. Pump wavelength 578.73 nm; (b) appearance of structure in the 830 nm bound-free spectrum.

two photon transition at 578.73 run, a broadband emission around 830 nm (fig. 2a) emerges from the heat pipe, collinear with the pump beam. In some cases a fine, reproducible structure persists, riding on a broad pedestal from 827 nm to 832 nm, see fig. 2b. The appearance of the continuous spectrum is an indication that the 830 nm emission is aboundfree transition as predicted by Konowalow and Julienne [lo] . The relatively reproducible interval of the fine structure reflects the rotational distribution of the b 32+ excited state population. Although the cross sectior?for stimulated emission is only an insensitive function of the upper level rotational quan turn number J’, the laser gain coefficient depends on the J’ population distribution [ 121. Knowledge of the vibrational and rotational constants of the upper bound b 3Z: state is needed in order to assign this structure. The ir superradiant emission is highly directional.

We have measured the intensity distribution of the signal, copropagating with the pump beam with a pin hole diaphragm, 15 cm from the output window. The result is plotted in fig. 3. The divergence measured in this way is 4 mrad. Of the same order of magnitude is also the divergence of the ir signal, situated closely at 840.24 nm, fig. 2, accompanying always the extimer emission. The narrow spectral width and similar characteristics (see also tigs. 4a,b) of this line sug gest a bound-bound transition from higher lying states. Roth emissions are closely coupled in the cascading process, involving probably also other emissions out of the wavelength range, measured with the available apparatus. In this way several J’ rotational levels of b 3Z+(u’ = 0) are populated. Using a dichroic !rirror we have tried to record a signal at 830 nm, travelling opposite to the pump direction. Within the OSA sensitivity there was no backward emission. In previous measurements [ 131 we have shown that under short pulse (2 ns) excitation, bound-bound emission is also mainly forward propagating. The high gain in the direction of pump without cavity mirrors can be understood by some kind of travelling wave under short pulse excitation. Gain anisotropy with a higher forward amplification is a well established experimental characteristic of the dimer lasers [2-4] . This feature of the emission might rise the question whether it is not caused by parametric or Raman process in the atomic or molecular system alone. First, parametric or Raman emission in atoms or mole.

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cules should be tunable when the pump wavelength is tuned. Our measurements show that 830 nm and 840.24 nm laser radiation have fixed wavelength independent of pump wavelength. Within the dye laser linewidth, they are excited only at the 3S-4D resonance. Second, an ir four wave parametric emission, from energetic considerations should be coupled with a signal wave at 444 nm. An intensive search in the range 300-900 nm gave some spontaneous lines around 480 nm, but no signal at 444 nm. The nearest atomic transition 3D-3P is at 818/819 nm [15,16]. Third, the density dependence of a parametric or Raman process should be exponential [ 171, in contrast to the slow dependence, measured here (see below). The broadband (5 nm) emission is also not a bound-free A1zi--XIZi transition to the repulsive wall of the ground singlet state (“Millikan’s difference potential”) [5]. Spontaneous fluorescence of this type was studied both theoretically and experimentally in Na, [5] and laser emission was recently reported with Kr+ laser pumping [6] . This emission, however is accompanied with much stronger boundbound lines [5,6] . What is also important, it should be excited by a number of lines with comparable intensities, due to transitions with nearly equal FranckCondon factors for slightly different J” within a given u” --f LJ’excitation band [ 12,181. In contrast, in our case the emission at 830 nm and the satellite 840.24 nm emission was excited only by the 3S-4D transition in the range studied, where numerous X ‘zi + A ‘z: absorption lines do exist [ 131. Our conclusion is, that the measured broadband emission at 830 nm is the bound-free b 3 ~+(IJ’ = 0) + x ‘2: excimer transition, as discussed in !ec. 3. Excitation with longer pulses will allow the introduction of an optical cavity and eventually, generation on weaker shorter wavelength bound-free transitions from b 3 Z:(u) = 1- 10) levels. An experiment of this type is in preparation now with a wavelength selective cavity in order to study the tuning possibilities ‘of the excimer emission. The stimulated character of the excimer emission is also clearly demonstrated by its intensity dependence, fig. 4a, taken at 590 K, 625 K and 660 K. The onset of stimulated amplification is clearly seen on the figure at pump energy of -60 fl. We show the signal intensity as a function of tem202

1 December 1984

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Fig. 4. Dependence of 830 nm and 840.24 nm signal intensity on pump laser energy (a), and temperature and sodium atom and molecular density (b).

perature and sodium density in fig. 4b. Since the extimer transition is to the lower lying repulsive part of the collisional x ‘Zi state (fig. l), increasing of T (and NNa) statistically populates this part of the curve. In this way the free-bound self absorption of the extimer wavelength is also increased. The coIlisiona1 population of the upper and lower laser levels are competing processes with approximately equal concentration dependence [ 121. Therefore, Za3u(NNa) is a relatively slowly varying function, as seen from fig. 4b. Note, that the optimum sodium atom density

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is 6.6 X 1014 cm-3 (T = 603 K). In contrast, boundbound dimer emission usually require higher optimum operation temperature. We show for comparison in fig. 4b (dashed line) the temperature dependence of A-X laser lines at X = 800 nm, recorded at the same experimental conditions by X-A pumping at 597 nm. Balance, mainly between gain and absorption causes the slope of the dependence over 630 K. In conclusion, we have observed a broadband bound-free emission in Na,, centered at 830 nm, assigned as b 3Z+(u’ = 0) + x 3 Zz excimer transition. A mechanism for population of the upper triplet b 3Ec+state is discussed, involving two-photon excitatioi of Na(4D) and collisional energy transfer to dimer triplet states. By measuring the spontaneous fluorescence, this type of energy transfer can be employed for determination of the triplet states’ molecular constants.

References [l] Excimer lasers, ed. Ch.K. Rhodes (Springer, Berlin, 1979). [2] B. Wellegehausen, IEEE J. Quantum Electr. 15 (1979) 1108. [3] W. Luhs and B. WelIegehausen, Optics Comm. 46 (1983) 121. [4] C.N. Man and A. Brillet, Optics Comm. 45 (1983) 95.

1 December 1984

[S] K.K. Verma, J.T. Bahns, A.R. Rajaei-Rizi and W.C. Stwalley, J. Chem. Phys. 78 (1983) 3599; H. Kato, T. Matsui and C. Noda, J. Chem. Phys. 76 (1982) 5678. [6] J.T. Bahns, K.K. Verma, A.R. Rajaei-Rizi and W.C. Stwalley, Appl. Phys. Lett. 42 (1983) 336. (71 E.J. Breford and F. Engelke, Chem. Phys. Lett. 53 (1978) 282; 71 (1979) 1994; E.J. Breford, Laserspektroskopische Untersuchung von Stijrungen in Alkalidimeren: NaK und Pbs, Dissertation, Unlversitiit Bielefeld 1982. [ 81 J. McCormack and A.J. McCaffery, Chem. Phys. Lett. 64 (1979) 98. [9] H. Kato and C. Noda, J. Chem. Phys. 73 (1980) 4940. [lo] D.D. Konowalow and P.S. Julienne, J. Chem. Phys. 72 (1980) 5815. [ 1 l] D.D. Konowalow, ME. Rosenkranz and M.L. Olson, J. Chem. Phys. 72 (1980) 2612. [ 121 H. Mies and P.S. Julienne, IEEE J. Quantum Electr. QE-15 (1979) 272. [ 131 S.G. Dinev, I.G. Koprinkov and I.L. Stefanov, to be published. [ 141 M. Allegrini, G. Alaetta, A. Kopystynska, L. Moi and G. Orriols, Optics Comm. 22 (1977) 329. [ 151 A.R. Striganov and N.S. Sventitskii, Tables of spectral lines of neutral and ionized atoms (Plenum, New York 1968) p. 231. [ 161 S. Bashkin and J.O. Stoner Jr., Atomic energy levels and grotian diagrams (North-Holland, Amsterdam, 1975) Vol. 1, p. 342. [ 171 A.V. Smith and J.F. Ward, IEEE J. Quantum Electr. QE-17 (1981) 525; W. Hart& Appl. Phys. 15 (1978) 427. [ 181 L.K. Lam, A. Gallagher and MM. Hessel, J. Chem. Phys. 66 (1977) 3550.

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