LiZn and LiCd vapour mixtures prepared in the heat-pipe oven are irradiated with pulsed excimer laser emission at 308 nm. Temporal evolution of the fluorescence spectra is observed showing that different processes occur such as multiphoton excitation, collisional energy transfer, photochemical reaction, relaxation etc. Temporal change of the LiZn and LiCd blue-green diffuse bands spectral shape is observed and interpreted in terms of previous spectral simulations. The effective lifetimes of different spectral features are determined.Mješavine para Li-Zn i Li-Cd pripremili smo u toplovodnim pećima i obasjavali pulsnim excimerskim laserom na 308 nm. Opažanje vremenskog razvoja fluorescentnih spektara pokazalo je postojanje različitih procesa kao što je multifotonska pobuda, sudarni prijenos energije, fotokemijska reakcija, relaksacija itd. Opaženu vremensku promjenu spektralnog oblika plavo-zelenih difuznih vrpci LiZn i LiCd molekula objasnili smo pomoću ranijih svojstava spektralnih simulacija. Odredili smo efektivna vremena života različitih spektralnih pojava

Azinović, Davorka

Gogić, Srećko

Labazan, Irena

Milošević, Slobodan

English

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ISSN1330–0008CODENFIZAE4TEMPORAL EVOLUTION OF INTERMETALLIC DIFFUSE BANDSPECTRAIRENA LABAZAN, DAVORKA AZINOVIĆ, SREĆKO GOGIĆ and SLOBODANMILOŠEVIĆInstitute of Physics, P.O.Box 304, HR-10000 Zagreb, CroatiaReceived 10 December 1999; Accepted 24 May 1999LiZn and LiCd vapour mixtures prepared in the heat-pipe oven are irradiated withpulsed excimer laser emission at 308 nm. Temporal evolution of the fluorescencespectra is observed showing that different processes occur such as multiphoton exci-tation, collisional energy transfer, photochemical reaction, relaxation etc. Temporalchange of the LiZn and LiCd blue-green diffuse bands spectral shape is observedand interpreted in terms of previous spectral simulations. The effective lifetimes ofdifferent spectral features are determined.PACS numbers: 33.10.-n, 33.20.Kf, 33.80.Gj UDC 539.194, 539.196Keywords: LiZn – LiCd vapour mixtures, pulsed excimer laser irradiation, temporal evo-lution of fluorescence spectra, multiphoton excitation, collisional energy transfer, photo-chemical reaction, relaxation, effective lifetimes1. IntroductionSpectra from intermetallic vapour mixtures are usually quite complicated dueto the overlap of different spectral features [1]. A general remedy to this problemis a careful preparation of the alloy composition [2–4]. LiZn and LiCd diffuse bandspectra were first reported in Refs. 5 and 6. These experiments were performedwith the pulsed laser but only time-averaged spectra were recorded. Subsequentdetailed spectral simulations have shown complex nature of the diffuse band spectra[7]. Both bound-bound and bound-free types of transitions were found to play animportant role.When pulsed lasers are used for excitation, different spectral features can bedistinguished by performing temporal analysis of spectra with nanosecond resolu-tion [8]. The temporal analysis of alkali-IIB excimer bands was used in the case ofFIZIKA A (Zagreb) 8 (1999) 1, 1–10 1labazan et al.: temporal evolution of intermetallic diffuse . . .NaCd and NaHg to identify molecular formation channel and relevant rate coeffi-cients [9,10].In this work we have studied temporal evolution of intermetallic diffuse bands,particularly of the LiZn and LiCd systems produced photochemically in the reac-tion:Li2(21Πu) + IIB −→ Li IIB(22Π) + Li , (1)where IIB is Zn or Cd atom. Under conditions of dense vapour mixtures reaction(1) is followed by the thermalization of the nascent population distribution of the22Π stateLi IIB(22Π, v′J ′) + IIB −→ Li IIB(22Π, v′∗J ′∗) + IIB (2)which is responsible for the change in the diffuse bands spectral shapes.2 4 6 8 10 12ERXQGIUHHERXQGERXQG22Π12Π42Σ+52Σ+32Σ+22Σ+12Σ+LiCd4 6 8 10 12 14 16 1801234308 nmEnergy (104  cm-1)Li (32P)Li (32D)Li (42D)Li (32S)Li (22P)Li (22S)Internuclear distance (Bohr)21Σu+C1ΠuB1ΠuLi2A1Σu+X1Σg+Fig. 1. Partial potential energy diagram of Li2 and LiCd molecules showing 308 nmexcitation, photochemical reaction path, relaxation process and LiCd bound-freeand bound-bound type of emission.We use excimer laser at 308 nm to excite Li2(C1Πu) and temporal analysis ofdifferent spectral features to clarify excitation processes and photochemical produc-tion of LiZn and LiCd molecules. Illustration of the processes (1) and (2) is shownin Fig. 1 for the LiCd case.2 FIZIKA A (Zagreb) 8 (1999) 1, 1–10labazan et al.: temporal evolution of intermetallic diffuse . . .2. ExperimentThe experimental arrangement was similar to that described in our previouspaper [8]. Metal vapour mixtures were generated in a heat-pipe ovens. The heat-pipe ovens were filled with approximately same amounts of lithium and zinc orcadmium. The temperature in the central region of the heat-pipe oven was variedfrom 1020 K to 1220 K. Helium or argon were used as buffer gases at pressures from0.67 to 13.3 kPa (5 to 100) Torr. The heat-pipe mode of operation is established forzinc or cadmium vapour. The preparation of electronically excited Li2 moleculesin specific rovibrational levels of the 2(C)1Πu state is achieved by means of the 20ns pulsed XeCl excimer laser lines at 308 nm (LPX 105E). Laser beam (initiallyof shape 2x1 cm) was spatially filtered and made parallel with a quartz telescope,having the diameter of 2-3 mm in the center of the heat-pipe oven. The horizontallaser induced fluorescence image was rotated by a Dove prism and focused to thevertical entrance slit of the monochromator. The resolution of the system was ofabout 0.1 nm. The signal from the photomultiplier with S20 cathode was averagedby means of a box-car averager (Stanford SR245). The analog output from the box-car was converted to digital signal through an A/D converter and fed to a laboratorycomputer. Temporal analysis is performed by means of a box-car averager or a 8-bit, 150 MHz digital storage oscilloscope (Hameg 1507). Spectral response of ourdetection system was measured by using a standard calibrated tungsten lamp andis found flat in the region of interest (450-550 nm).We performed two types of measurements. To measure laser induced fluores-cence we scan the monochromator in wavelength interval of interest, setting a spe-cific gate and time delay value of the box-car averager. Time resolved intensity ofthe fluorescence is measured at a given setting of the monochromator by using adigital storage oscilloscope or by scanning the time delay of a box-car gate. Zero ofthe time scale is set to the rising edge of the laser pulse.3. Results and discussion3.1. Excitation of Li-Zn vapour mixtureFigure 2 shows the fluorescence of the LiZn vapour mixture in the region from420 to 500 nm, excited by the laser lines at 308 nm. For short time delay of10 ns (Fig. 2a) we observed only the LiZn excimer band (22Π − 12Σ transition),peaked at 477 nm [5]. At the delay of 40 ns we noticed Zn atomic lines at 472.2and 481.2 nm (Fig. 2b) (53S1 - 43P1 and 53S1-43P2 transitions). For the delay of70 ns (Fig. 2c) Zn lines decreased substantially. This time behaviour suggeststhat Zn atoms are populated in near resonant two-photon absorption processZn(41S0) + 2hν+M−→Zn(63S1)+M, where M is Zn or Li atom, followed by aquick cascade emission Zn(63S1) −→Zn(53P1) −→ Zn(53S1). Proposed excitationmechanism of zinc atomic emission is illustrated in Fig. 3. Alternative excitationmechanism which involves energy pooling collision 4p 3PJ + 4p3PJ can be ex-cluded on the basis of temporal evolution [12]. Energy pooling excitation resultswith microsecond decay times. As zinc atomic features decrease, Li atomic linesFIZIKA A (Zagreb) 8 (1999) 1, 1–10 3labazan et al.: temporal evolution of intermetallic diffuse . . .at 460.3 and 497.2 nm appear (4D-2P and 4S-2P transitions) in spectra. Lithiumatomic spectral features are consequence of the dissociative recombination of Li+2 ,the latter being produced via Li2(C1Πu) + hν −→Li+2 + e process.0,01,02,03,0 a) ∆=10 ns  G=10 ns 0,02,04,06,0 b) Zn(53S1-43P1) Zn(53S1-43P2)∆=40 ns  G=10 ns0,20,40,6I (arb.units)c)Li(42S-22P)Li (42D-22P)∆=70 ns  G=100 ns420 430 440 450 460 470 480 490 5000,00,20,40,60,8λ (nm)LiZnd)Zn(53S1-43P1)Zn(53S1-43P2)Li(42S-22P)Li (42D-22P)∆=1 ns  G=300 ns012345678virtual level308 nmvirtual level308 nm4p8s 7p6p7s468.1, 472.3,4s6s5s5p4p5d4d7s6s5s6p5p5d4d481.23D1,2,33P0,1,23S11S01P11D2Energy (104   cm-1)Fig. 2. The spectra of the Li-Zn vapour mixture excited by 308 nm laser lines at Arpressure of 6.9 kPa (52 Torr) and temperature T =1050 K. Time delays and gatesare: a) ∆ = 10 ns, G = 10 ns, b) ∆ = 40 ns, G = 10 ns, c) ∆ = 70 ns, G = 100 nsand d) ∆ = 1 ns, G = 300 ns.Fig. 3 (right). Term diagram of zinc atom illustrating excitation mechanism of ahigh-lying levels by 308 nm photons. Data are taken from Ref. 11. The energydefect for the first step is about 30 cm−1 and for the second step 490 cm−1.Figure 2d shows spectrum with large gate of the box-car averager which shouldbe compared with the time averaged spectra (see Ref. 5). Following the sequencefrom a to c, we observe change of the LiZn band shape. The band tail extendingfrom 470 nm to 440 nm, visible also in the time-averaged spectrum (Fig. 2d), ismost pronounced for earliest time delays. The ratio of intensities at band maximum(477 nm) and band tail (455 nm) changes from 1/4, 1/7 to 1/14. According tospectral simulations presented in Ref. 7 this long tail is of bound-free nature andoriginates from higher vibrational states of the 22Π state (v’≥ 7). Therefore, theobserved change in the band shape indicates relaxation of the nascent populationdistribution which is obviously centered at high rovibrational levels of the 22Π state.4 FIZIKA A (Zagreb) 8 (1999) 1, 1–10labazan et al.: temporal evolution of intermetallic diffuse . . .123456b) at 339.4 nm   Li2I (arb. units)12345a) at 477.5 nm    LiZn0 20 40 60 80 100 120012345t (ns)c) at 460.3 nm   LiFig. 4. Temporal evolution at a) LiZn band (477.5 nm), b) Li2C1Πu-X1Σ+g band(339.4 nm), and c) Li42D-22P atomic line (460.3 nm). Temperature was T = 1050K and argon buffer gas pressure 8.4 kPa (11 Torr). The full curve represents adouble-exponential fit of the experimental values.The temporal evolutions at a) 477.5 nm (LiZn), b) 339.4 nm (Li2 C state) andc) 460.3 nm (Li) are shown in Fig. 4. We have determined the effective lifetimesof the corresponding emissions, which are at T = 1050 K and (12 ± 5) ns forLiZn(22Π), (10 ± 3) ns for Li2(21Πu) and (19 ± 3) ns for Li(42D) state. Withincreasing temperature the metal vapour pressure increases causing the LiZn(22Π)effective lifetime to decrease linearly, i.e. at 990 K τeff = (20± 5) ns and at 1060K τeff = (8.5 ± 4) ns. The lifetime is approximated by the extrapolation to thezeroth zinc vapour pressure at 723 K, which gives τrad = (65± 26) ns.3.2. Excitation of Li-Cd vapour mixtureFigure 5 shows spectra of the Li-Cd vapour mixture excited by 308 nm laserlines at T = 1030 K and a) ∆ = 25 ns and G = 10 ns, b) ∆ = 50 ns and G = 10ns, c) ∆ = 70 ns and G = 75 ns and d) time-averaged ∆ = 10 ns and G = 200 ns.Excited LiCd(2 2Π) molecules are produced through the photochemical reaction[7] with nascent rovibrational population distribution far from Boltzmann distribu-tion (see Fig. 1), which gives dominant bound-free emission for short time delaysFIZIKA A (Zagreb) 8 (1999) 1, 1–10 5labazan et al.: temporal evolution of intermetallic diffuse . . .after the laser pulse (Fig. 5a). Subsequent collisions with Cd atoms are efficientlythermalizing the nascent rovibrational population. This is observed, for larger timedelays (see Fig. 5b and 5c), as the change of spectral shape of the blue-green diffuseband, with dominant bound-bound contribution to the emission [7] (sharp peaksnear the band maxima). In the present experiment we do not observe cadmiumatomic lines in the spectral region shown in Fig. 5, neither the characteristic emis-sion at 326.1 nm (5p 3P1- 5s1S0 transition). The latter was observed in Ref. 13for Cd-He mixtures where 308 nm excitation was explained to proceed through thecharacteristic features of the Cd-He collision complex. Although one could expectrather similar structure of the Cd-He and Cd-Ar or Cd-Li complexes they are quitedifferent in respect to the efficiency of specific collisional energy transfer process.This shows that careful preparation of metal mixtures and choice of a buffer gascan be used to efficiently promote different spectral features. Lithium atomic linesappear after 70 ns similarly as in the LiZn case. We note that, as in LiZn case,450 460 470 480 490 5000,00,51,0I (arb. units)d)  ∆=10 ns, G=200 nsλ (nm)0,00,5a)  ∆=25 ns, G=10 ns0,5c)  ∆=70 ns, G=75 ns0,02,0b)  ∆=50 ns, G=10 nsFig. 5. The spectra of the Li-Cd vapour mixture excited by 308 nm laser lines atAr pressure of 13 kPa (100 Torr) and temperature T = 1030 K for different sets[(a) to (d)] of time delay and gate.Li2 spectral features (diffuse band at 458 nm) are absent from spectra. This is anindication of the low Li and Li2 density in the vapour mixture above liquid alloy.Figure 6 shows LiCd diffuse band spectra in the region 450 - 500 nm recorded at6 FIZIKA A (Zagreb) 8 (1999) 1, 1–10labazan et al.: temporal evolution of intermetallic diffuse . . .the time delay of 75 ns for different buffer gas (Ar) pressures. We assume that forall cases the heat-pipe works in the heat-pipe mode for cadmium, namely centralheated part mainly contains cadmium vapours with small portion of lithium vapour(few percent), while buffer gas fill space between heated zone and windows of theheat pipe. Therefore the change of the diffuse band shape is caused by collisionsgiven by process of type (2). For a low buffer gas pressures the diffuse band shapepreserve bound-free nature even at large time delays.450 460 470 480 490 5000,00,5I (arb. units)d)  P = 100 torrλ (nm)0,00,51,01,5a)  P = 5.6 torr0,00,5c) P = 70 torr0,00,5b) P = 20 torr050100150200250QP050100150200QP050100150200,QWHQVLW\DUEXQLWVQP050100 QP0 100 200 300 400 5000501001502007LPHQVQPFig. 6. The spectra of the Li-Cd vapour mixture excited by 308 nm laser taken withtime delay ∆ = 75 ns and gate G = 60 ns for different buffer (Ar) gas pressures.T = 1000 K.Fig. 7 (right). Temporal evolution of different spectral features from LiCd vapourmixture irradiated by 308 nm laser. From the top to bottom fluorescence is shownfor: two LiCd diffuse band maxima, 2P-2S lithium transition, Stokes emissionwithin C-X manifold and scattered light of the laser pulse. Temperature was 1040K and buffer gas pressure 7.4 kPa (5.6 Torr).Figure 7 shows the time evolution of different spectral features obtained bymeans of a digital oscilloscope. Set of such curves have been measured for differentbuffer gas pressures. From each curve decay times were determined by performingsingle-exponential fit starting at about 60 ns (falling edge of the laser pulse). This isFIZIKA A (Zagreb) 8 (1999) 1, 1–10 7labazan et al.: temporal evolution of intermetallic diffuse . . .done in order to avoid influence of the laser pulse width. Plotting effective radiationrates (inverse of decay time) versus Cd atom density allows us to determine thetotal quenching rate coefficients for relevant upper levels (see Fig. 8). Data for zerocadmium density are obtained from measurement in the pure lithium heat-pipeoven at the same temperature and buffer gas pressure. We note that emission from2P level is strongly influenced by radiation trapping. We obtain quenching ratecoefficient for the Li(2P) level to be (3±1) · 10−12 cm3s−1 and for the C 1Πu (v’≈13-19) Li2 levels (1.2 ±0.5)·10−11cm3s−1. The effective lifetimes of LiCd spectralfeatures at 489 nm and 482 nm were determined to be 15 ns and 14 ns, respectively.We did not observe significant change of the effective lifetimes with atom densityas in the LiZn case.     EΓ V1&GFP DΓ VFig. 8. The effective radiative rates of a) Li(2P) and b) Li2(C-X) band versuscadmium atom density for T = 1040 K. The natural radiative rate of Li(2P) is3.69 · 107 s−1 [14]. The natural radiative rate of the C-state is about 1.6 · 107 s−1[15].4. ConclusionsWe have shown that, in spite of the relatively large collision frequencies for totalpressures usually used within the heat-pipe ovens, nanosecond time spectral analy-8 FIZIKA A (Zagreb) 8 (1999) 1, 1–10labazan et al.: temporal evolution of intermetallic diffuse . . .ses can provide important information for understanding of complicated processesin intermetallic vapour mixtures. Photochemical production of the LiZn(22Π) andLiCd(22Π) excimers by means of 308 nm laser photons results with population ofhigh vibrational states. By performing temporal spectral analysis we observed theredistribution of the population towards the lower vibrational states which com-pletes after the time delays larger then 50 ns. The observed changes in the diffuseband shapes are consistent with predictions of detailed spectral simulations givenin Ref. 7. The thermalization effect is especially pronounced in the case of LiCdthrough appearance of bound-bound structures, which originate from lower vibra-tional levels in the upper state. Measured effective lifetimes of the LiZn and LiCdemission are similar as those reported for NaCd and NaHg excimers [9,10].AcknowledgementsThis work was financially supported by the Ministry of Science and Technol-ogy of the Republic of Croatia. We wish to thank Dr. Goran Pichler for valuablediscussions and continuous support of this work.References1) S. Milošević, in Spectral Line Shapes, Vol. 8, Eds. A. D. May, J. R. Drummond and E.Oks, AIP Conf. Proc. 328 (1995) pp. 391-405.2) D. Azinović, X. Li, S. Milošević, G. Pichler, M. C. van Hemert and R. Düren, J. Chem.Phys. 98 (1993) 4672;3) D. Azinović, S. Milošević and G. Pichler, Chem. Phys. Lett. 233 (1995) 477;4) D. Azinović, S. Milošević and G. Pichler, Z. Physik D 36 (1996) 147;5) S. Milošević, X. Li, D. Azinović, G. Pichler, M. C. van Hemert, A. Stehouwer and R.Düren, J. Chem. Phys. 96 (1992) 7364;6) M. C. van Hemert, D. Azinović, X. Li, S. Milošević, G. Pichler and R. Düren, Chem.Phys. Lett. 200 (1992) 97;7) X. Li, S. Milošević, D. Azinović, G. Pichler, R. Düren and M. C. van Hemert, Z.Phys.D 30 (1994) 39;8) D. Azinović, S. Milošević and G. Pichler, Fizika A 5 (1996) 167;9) D. Gruber, U. Domiaty, L. Windholtz, H. Jeager, M. Muso, M. Allegrini, F. Fuso andA. Winkler, J. Chem. Phys. 100 (1994) 8103;10) D. Gruber, U. Domiaty, K. Iskra, S. Dinev and L. Windholz, J. Phys. Chem. 100(1996) 7078;11) C. E. Moore, Atomic Energy Levels, Vol. II, NBS 467 (1952) p. 124;12) H. Umemoto, A. Masaki, J. Kikuma and S. Sato, Chem. Phys. 141 (1990) 457;13) H. Umemoto, T. Ohnuma and A. Masaki, Chem. Phys. Lett. 171 (1990) 357;14) W. I. McAlexander, E. R. I. Abraham and R. G. Hulet, Phys. Rev. A 54 (1996) R5;15) Th. Weyh, K. Ahmed and W. Demtröder, Chem. Phys. Lett. 248 (1996) 442.FIZIKA A (Zagreb) 8 (1999) 1, 1–10 9labazan et al.: temporal evolution of intermetallic diffuse . . .VREMENSKI RAZVOJ INTERMETALNIH DIFUZNIH SPEKTARAMješavine para Li-Zn i Li-Cd pripremili smo u toplovodnim pećima i obasjavalipulsnim excimerskim laserom na 308 nm. Opažanje vremenskog razvoja fluores-centnih spektara pokazalo je postojanje različitih procesa kao što je multifotonskapobuda, sudarni prijenos energije, fotokemijska reakcija, relaksacija itd. Opaženuvremensku promjenu spektralnog oblika plavo-zelenih difuznih vrpci LiZn i LiCdmolekula objasnili smo pomoću ranijih svojstava spektralnih simulacija. Odredilismo efektivna vremena života različitih spektralnih pojava.10 FIZIKA A (Zagreb) 8 (1999) 1, 1–10

Vremenski razvoj intermetalnih difuznih spektara

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