Spectroscopic diagnostics of lasersupported absorption (LSA) waves produced by aHF laserP. S. P. Wei and D. B. Nichols Citation: Journal of Applied Physics 47, 3054 (1976); doi: 10.1063/1.323051 View online: http://dx.doi.org/10.1063/1.323051 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/47/7?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Laser-supported ionization wave in under-dense gases and foams Phys. Plasmas 18, 103114 (2011); 10.1063/1.3642615 Lasersupported detonation waves and pulsed laser propulsion AIP Conf. Proc. 208, 359 (1990); 10.1063/1.39425 An interferometric investigation of laser−supported absorption waves J. Appl. Phys. 46, 761 (1975); 10.1063/1.321642 Experimental studies of lasersupported absorption waves with 5ms pulses of 10.6μ radiation J. Appl. Phys. 44, 3675 (1973); 10.1063/1.1662819 Emission spectra of lasersupported detonation waves J. Appl. Phys. 44, 2311 (1973); 10.1063/1.1662555

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Spectroscopic diagnostics of laser-supported absorption (lSA) waves produced by a HF laser*

p. s. P. Wei and D. B. Nichols

Boeing Aerospace Company. Seattle. Washington 98124 (Received 22 January 1976; in final form 17 March 1976)

LSA waves have been generated by a pulsed chemical HF laser beam (2.8 Ji. 45 J. 7 Jisec) when focused on solid surfaces in air. The emission spectrum is found to contain mainly N+ and 0+ lines broadened by the Stark effect and superimposed on a strong continuum. The LSA wavefront travels at an average speed of 0.72 cm/Jisec as measured by the time-of-flight method. Target species such as atomic Al emit light for 600 Jisec after the laser pulse. In contrast with the results from a CO2 laser, however, no forbidden transition of AI I is observed.

PACS numbers: 52.50.Jm, 52.70.Kz, 52.35.Lv, 79.20.D8

Theoretical aspects of igniting a traveling laser spark at beam intensities much below the gas breakdown threshold have been considered by Raizer.1,2 Experi­ments on LSA waves, mainly produced by CO2 lasers from solid surfaces, have been reported. 3-14 Once ig­nited, an LSA wave absorbs further laser radiation and propagates towards the laser source. Thus, the solid surface may be totally or partly shielded for a while. When the laser intensity decreases, the LSA wave cools down and becomes less effective in shielding. Then, the laser light may reach the surface again. The ab­sorption of high-power laser radiation by a semiopaque material gives rise to localized heating at the surface. 15 At power densities below about 107 W /cm2, the evapora­tion of the surface can be described by a thermal model. 16

In this paper we report some new results on HF­laser-produced plasmas as studied by time-resolved spectroscopy. Although the LSA waves are very simi­lar to those produced by CO2 lasers, the laser-target coupling turns out to be quite different due to the differ­ent absorbance at 2.8 and 10.6 Il for many organic17

and inorganic18 substances. For example, sapphire (AI20 3) and quartz (Si02) are semiabsorbing at 10.6 Il but are both transparent for X < 7 Il. As a consequence, we have not been able to generate an intense plasma plume of Al or Si in the present experiments. Not much data on HF-Iaser beam-target interaction have been reported except for an experiment on impulse coupling. 13

The photoinitiated pulsed chemical HF laser, 20 as pumped by four Xe flash lamps, yields pulses of 15-MW peak power in 7 Ilsec, with a FWHM of about 3 Ilsec. Most of the laser power is distributed among about 5 lines between 2.7 to 3.0 Il. Spectra of HF laser emission have been studied in a previous experiment in our laboratory. 21 When the lO-cm-diam. beam is focused down with a mirror ({=37.5 cm), about 45 J of energy is deposited in a circular area of 1. 6 cm in diameter. About 69% of the deposited energy is further concentrated within an ellipse of semiaxes 2 by 3 mm, which yields a maximum power density of 5.5 X 107 W / cm2 on the target. More details about the laser and related experiments are described elsewhere. 20

Time-integrated emission spectra are recorded with the Hilger-Watts quartz-prism spectrograph. 6 A near-

3054 Journal of Applied Physics. Vol. 47, No.7, July 1976

normal-incidence 1-m concave-grating monochromator (McPherson model 225) is used to monitor the temporal behavior of spectral lines. With a quartz lens focused at a distance z in front of the target surface, light emitted within a thin slice of space at z can be imaged onto the entrance slit. Removing the lens, the solid angle subtended by the monochromator contains the entire spark including z = O.

Figure 1 shows three sections of the time-integrated spectra observed 4 mm in front of an Al plate. With the deposited laser energies of 43 and 34 J, the spec­trum is dominated by N+ and 0+ lines superimposed on a strong continuum. These lines are more broadened and shifted by the Stark effect than in a similar spec­trum generated by a CO2 laser. 6 In contrast to the air species, atomic lines from Al are narrow and they serve as a calibration of the wavelength scale. At 21 J, Al lines are the only emission feature recorded. The

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FIG. 1. Sections of the emission spectra observed at 4 mm in front of an AI-plate target. The deposited laser energy is indicated in the parentheses.

Copyright © 1976 American Institute of Physics 3054

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>­l-

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FIG. 2. The LSA wave continuum emission observed at sev­eral distances in front of the target. The arrows designate the onset of light. A laser pulse shape is included for comparison.

maximum power density of 2.6 x 107 W /cm2 indicates a threshold for the air breakdown which is in agree­ment with the observation by Marcus and Lowder. 19

A shot at 11 J does not produce enough light to be recorded.

According to the theory on spectra line broadening, 22

singly charged ions are mainly broadened by electron impact. In the bottom spectrum of Fig. 1, the widths for the N+ 3007 and 3328 lines are measured to be 17 ± 1 and 26 ± 1 A, respectively. The line widths as cal­culated from the impact approximation22 are directly proportional to the electron density 11., but are not very sensitive to the temperature. Thus, with an estimated temperature5 of 20000~, the 11. is 8.4±0.4 x 1018/cm3•

Similarly, for the middle spectrum in Fig. 1, 11. is estimated to be 4.1±0.3 x I018/cm3• Note that the time­integrated spectrum yields an average width only. Furthermore, for the crude estimates above, the Doppler width « O. 04 A) and the instrumental width « O. 9 A) are both neglected.

Metallic plates such as AI, Ti, and Cu often exhibit visible damages after the laser irradiation. But only a small fraction of the target vapor is heated to high enough temperatures to contribute to the emission. Even for a piece of anodized black aluminum foil, the number density of Al ions in the plasma is not high enough to produce measurable np and nf forbidden transitions in Al as observed previously in the CO2 laser experiments. 23,24

It is of interest to study the time of arrival of the LSA wavefront. Figure 2 shows some typical traces of the continuum observed at several distances. A laser pulse shape monitored by a liquid-N2-cooled

3055 J. Appl. Phys., Vol. 47, No.7, July 1976

Ge(Au) detector is included for comparison. The os­cilloscope sweeps are synchronously triggered by the current pulse of the flash lamps. The onset of light emission is designated by arrows. We find that the time-of-flight may be fitted by a straight line of slope 0.72 cm/fJ.sec, which represents the average speed of the wavefront. This is about twice as fast as that mea­sured in the CO2-laser experiments6 of comparable power density but smaller focal area (2.5 mm in diam­eter). A faster wavefront means higher density, and hence more broadening of spectral lines. The LSA wave proceeds to about z = 2 cm and then decays with the laser pulse. In general, the laser absorption coef­ficients in air depend on the pressure, the temperature, and the laser photon energy. Raizer has given some numerical estimates for the cases of a Nd laser! and for a CO2 laser. 2 The threshold for plasma production also depends on the power level, the pulse duration, the focusing geometry, and the target. Thus, the present results should not be directly compared with those from the CO2 laser. 6

Figure 3 shows the traces of several emission lines obtained one at a time from the plasmas generated from an Al plate. The deposited laser energy is fairly repro­ducible; in ten shots the average deviation is less than 6%. The quartz lens is removed so that the mono­chromator views the whole luminous event. The AI" trace (not shown) is slightly different from the continu­um in that the AI++ decays more gradually between 20 and 40 fJ.sec. In fact, for all the lines studied, the con­tinuum is so strong that it dominates the Signal in the

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FIG. 3. Temporal behavior of four kinds of emission lines from the plasma produced by a 45-J pulse from an Al plate.

P.S.P. Wei and D.B. Nichols 3055

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first 10 Ilsec. The temporal behavior of emission lines indicates that Al ions relax through connecting stages of ionization followed by reaction with air, which is simi­lar to the relaxation of an electric spark. 25 We see that the excited Al atoms and AIO molecules emit light for 600 and 800 11 sec, respectively, after the laser pulse. The ground- state atoms and molecules ought to survive even longer. The plasma relaxation time, which is much longer than the nanosecond decay time of the ex­cited states, may be important in a multiple-pulse experiment. Laser attenuation by an aluminum vapor has been observed by Bonch-Bruevich et al. 26

An interesting observation is that the time trace for Al' exhibits two maxima separated by - 9 Ilsec which can be correlated with the Al trace with two maxima separated by -16 Ilsec. This may be understood in terms of the plasma-shielding effect. Evaporation oc­curs whenever the laser light can bring the surface to its boiling point. This is true during the initial onset and the final decay parts of the laser pulse. Near the peak of the pulse, on the other hand, the air breakdown plasma shields the target so that the evaporation is re­duced. This is in agreement with calculations by Jackson. 27

The detailed mechanism for laser-solid coupling near the threshold of surface-induced breakdown is very complex. Further experiments are needed to fully as­sess the important characteristics at the HF-Iaser wavelength.

The authors wish to thank Dr. R. B. Hall for sug­gesting the study, and to thank D. C. Botz and L. Alexander, Jr., for their capable technical assistance.

lyU. P. Raizer, JETP Lett. 7, 55 (1968). 2yu. P. Raizer, Sov. Phys.-JETP31,1148(I970). 3V.A. Batanov, V.A. Bogatyrev, N.K. Sukhodrev, and V.B.

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Fedorov, Sov. Phys. -JE TP 37, 419 (1973). 4A.I. Barchukov, F. V. Bunkin, V.I. Konov, and A. M. Prokhorov, JETP Lett. 17, 294 (1973).

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Phys. 44, 3675 (1973). 13E. L. Klosterman and S. R. Byron, J. Appl. Phys. 45, 4751

(1974). 14D. C. Smith and R. T. Brown, J. Appl. Phys. 46, 1146

(1975). 15J. F. Ready, Effects of High-Power Laser Radiatian

(Academic, New York, 1971). 16p. S. P. Wei, D. J. Nelson and R. B. Hall, J. Chern. Phys.

62,3050 (1975). 171njYared Spectroscopy, Its Use in the Coating Industry,

edited by L.C. Afremow (Fed. Soc. Paint Technology, Philadelphia, Pa., 1968).

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19S. Marcus and J. E. Lowder, J. Appl. Phys. 46, 2293 (1975).

2oR. B. Hall, W. E. Maher, J. D. McClure, D. B. Nichols, C.R. Pond, and P.S.P. Wei, Final Report on Air Force Weapons Laboratory Contract F29601-73-A-0038-0002 (1975) (unpublished).

21D.B. Nichols, K.H. Wrolstad, and J.D. McClure, J. Appl. Phys. 45, 5360 (1974).

22H. R. Griem, Spectral Line Broadening by Plasmas (Academ­ic, NewYork,1974), Chap. 2, p. 371.

23P.S.p. Wei, K.T. Tang, and R.B. Hall, J. Chern. Phys. 61, 3593 (1974).

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A. S. Zakharov, V. N. Kotylev, and 0.1. Kalabushkin, JETP Lett. 17, 241 (1973).

27J. P. Jackson (private communication).

P.S.P. Wei and D.B. Nichols 3056

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