Measurement of Photoionization Cross Section of Sodium

Transcription

1 Org. Opto-Elect., No. 1, 15-0 (016) 15 Organo Opto-Electronics An International Journal Measurement of Photoionization Cross Section of Sodium M. Atif 1,,* 1 Physics and Astronomy Department, College of Science, King Saud University, Riyadh, Saudi Arabia. National Institute of Laser and Optronics, Nilore, Islamabad, Pakistan. Received: 1 Feb. 016, Revised: Mar. 016, Accepted: 4 Mar Published online: 1 Jul Abstract: An atomic beam apparatus has been used to observe the sodium spectrum using two-step laser excitation schemes. An R6G dye laser pumped by an excimer laser 308 nm excites the atom from the 3s S 1/ ground state to the 3p P 1/ or 3p P 3/ multiplets of the first excited state which are further ionized by a photon from the 308 nm of the Excimer laser. The ions are directed to the channeltron by applying a negative voltage. The ions signal from the channeltron has been recorded on a computer after processing through a box car averager. The absolute value of the photoionization cross sections for Na has been measured as Mb The present measured value of photoionization cross sections are in excellent agreement with the existing experimental and theoretical work. Keywords: atomic beam apparatus, photoionization cross-sections, excimer laser, channeltron. 1 Introduction A number of theoretical as well as experimental techniques have been established for the measurement of the photoionization cross section of the ground and the excited states of atoms and molecules. Various methods have been developed by a number of researchers around the world for the determination of photoionization cross sections of atomic states (Letokhov 1987 and references therein) [1-0]. Another technique like the saturation of measuring the photoionization cross section was first investigated by Ambartzumian et al (1976). This method has been used widely by many Scientists (Nygaard et al 1975, Bradley et al 1976, Heinzmann et al 1977, Smith et al 1980, Burkhardt et al 1988, Kallenbach et al 1988, He et al 1991, 1995, Xu et al 1993, Willke and Kock 1993, Mende et al 1995, Amin et al 006a,b and Saleem et al 006a,b, Hussain et al 006 a, b Kalyar et al 007b) [-0] to measure the photoionization cross section of the excited states of alkali and alkaline earth metals and rare gas atoms. Mahmood et al 006 [1] has exploited this method for the photoionization cross section measurement and the optical oscillator strength of the autoionizing resonances in neon. Recently the photoionization cross section of sodium from the excited states has been reported by Baig et al 007 and Rafiq et al 008 [-3]. We shall describe here atomic beam technique to measure beam density and photoionization cross sections of sodium. Experimental Arrangement The experimental arrangement is shown in figure 1 consists of atomic beam apparatus [4] (vacuum chamber), a pulsed Excimer laser, Dye laser and a 100 Hz storage oscilloscope. First, we created a vacuum of the order of 10-6 torr by using a rotary pump and an oil diffusion pump in the vacuum chamber. The channeltron is installed for the detection of ions. The detection system is shown in figure. In order to form an atomic beam, we use an oven with two slits of diameter 30 mm and 0 mm at a distance 3cm each respectively. The oven is made with a ceramic tube of length 10 cm and diameter 1.95 cm having one end open and the other closed. The Nichrome wire is wounded on the ceramic tube about 8 cm in length for heating purposes. The S.S pellet is placed inside the ceramic tube of length 10 cm and diameter 1.95 cm to avoid heat loss and to maintain temperature constant. The ceramic tube is further shielded by wrapping the fiber glass wool. The oven is placed in the vacuum chamber and a sample of sodium was loaded in it. The sodium is heated in the oven at C by applying a voltage of 8 volts & a sample of sodium atom emerged through an aperture of 0.5 mm diameter into a high vacuum chamber. * Corresponding author atifhull@gmail.com

2 16 M. Atif: Measurement of Photoionization Cross Figure 1 Experimental setup for Spectroscopic Studies Figure Detection System A DC voltage ranging between -1.5 kv is applied to the channeltron. The experiment is performed by using two-step laser excitation schemes. The first dye laser pumped by the Excimer laser having ns pulse duration and Hz repetition rate (continuously adjustable) promotes the atom from the ground state to the resonance lines which are further ionized by a photon from the 308 nm Excimer laser. The observed signal is fed to boxcar averager model SR-50. Box car averager is defined as an electronic test instrument used to integrate signal input voltage after a defined waiting time (trigger delay) over a specified period of time (gate width) and then average over multiple integration results (samples). The signal is processed through a boxcar averager. The boxcar is synchronized with the laser. The output signal from the boxcar averager is finally fed to a computer to record the spectrum for subsequent analysis. 3 Results and Discussion A dye laser pumped by an excimer laser at 308 nm tuned 5890 A 0 and 5896 A 0 excites the atom from the 3s S 1/ ground state to the 3p P 1/ or 3p P 3/ multiplets of the first excited state which are further ionized by a second photon from the 308 nm of the excimer laser. The sodium spectrum has been taken at 50 o C, which is shown in figure 3. The ideal shape of the D1 and D lines including the energy level diagram for the ionization of sodium atom is reported in my previous publication. In our previous reported study of sodium spectrum from atomic beam apparatus, the peak has a sharp shape but in our present study the shape of peaks appearing different from the original peaks. The reason is that we apply a DC voltage of -1.5 kv is to the channeltron and an Aluminum plate is installed ungrounded at a distance of.5 cm. Due to this reason an electric field is appearing in the spectrum of the sodium atom. We have also observed another absorption peak at nm. The dye laser is tuned from nm and the data is recorded at three different temperatures shown in figure 4.

3 Org. Opto-Elect., No. 1, 15-0 (016) / Ionization Intensity (a.u) Wavelength (nm) Figure 3 Sodium spectrum taken at 50 o C. 0.5 Ionization Intensity (a.u) Wavelength (nm) Figure 4 Sodium spectrums at three different temperatures. The beam density and photoionization cross-section of Na are calculated as follows Cylindrical Tube: The divergence of the atomic beam is calculated as follows w w = 0.5mm L = 5mm L = (0.5)/5 = 0. = (0.180 o )/ = o The value of agrees very well with the value calculated from fig radian = 360 o Aperture: The number of particles per second effusing into a solid angle do at an angle with respect to normal to A s is nvas N( ) cosdw (1) 4 The total number of particles per second escaping through the aperture is obtained by integrating the equation (1)

4 18 M. Atif: Measurement of Photoionization Cross nvas N 4 n P kt And PAsNa N 1 MRT 8kT v m The number of particles per second striking a target of the unit area on the axis of aperture ( =0) at a distance L from the aperture. N I L L PAs 1 MT 1 particles/cm sec D = 0.5mm R = 0.5/ = 0.5mm R = 0.05cm A s = R = cm P = vapor pressure of Sodium atom = 10-3 mm Hg L = Distance from the aperture to the center of the parallel plates = 10cm M = molecular weight for Sodium = 3 T = 00 C+73 = 473 K I =.09x10 14 particles/cmsec K = 1.38x10-16 erg / K m = M /Avogadro s Number = 3 / 6. 03x10 3 Hence the beam density is = 3kT m The mean free path is related to the pressure P through the relation = cm 1 v = cm/sec I v = / = particles/cm 3 1 kt n P For the calculation of photoionization cross-section for Na 1J = (nm) photons The value of photon fluxes at three different energies = atomic diameter for Sodium = cm P = 10-3 mmhg = erg/cm 3 = 8.05 cm 34 8 hc E J

5 Org. Opto-Elect., No. 1, 15-0 (016) / 19 Photon flux = Energy density Pulse width F = photons cm - s -1 The rate of photoionization for Na = w = s -1 is taken from Wiese and Martin table. Using the formula w = F 4 Conclusion 1 = cm = cm 3 = cm The average value comes out to be = cm = Mb This is in excellent agreement with the Burkhardt s value [5]. = Mb The experiments have been performed using two-step photoionization ionization technique. The atomic beam density and photoionization cross-section of Na have been found by atomic beam technique. The absolute value of the photoionization cross sections from Na has been measured as Mb with atomic beam density particles/cm 3. The present calculated value of the photoionization cross sections is in good agreement with the existing experimental and theoretical value. Acknowledgement The authors would like to extend their sincere appreciation to the Deanship of Scientific Research at King Saud University for its funding of this research through the Research Group Project No. RGP-VPP-93. References [1] Letokhov V S 1987 Laser Photoionization Spectroscopy (Academic Press Florida) [] Ambartzumian R V, Furzikov N P, Letokhov V S and Puretsky A A 1976 Appl. Phys [3] Nygaard K J, Hebner R E, Jones J D and Corbin R J 1975 Phys. Rev. A [4] Bradley D J, Dudan C H, Ewart P and Purdie A F 1976 Phys. Rev. A [5] Heinzmann U, Schinkowski D and Zeman H D 1977 Appl. Phys [6] Smith A V, Goldsmith J E M, Nitz D E and Smith S J 1980 Phys. Rev. A 577 [7] Burkhardt C E, Libbert J L, Xu J, Leventhal J J and Kelley J D 1988 Phys. Rev. A [8] Kallenback A, Kock M and Zierer G 1988 Phys. Rev. A [9] He L- W, Burkhardt C E, Ciocca M and Leventhal J J 1991 Phys. Rev. Lett [10] He L-W, Burkhardt C E, Ciocca M, Laventhal J J, Zhou H-L and Manson S T 1995 Phys. Rev. A [11] Xu C B, Xu X Y, Ma H, Li L Q, Huang W, Chen D Y and Zhu F R 1993 J. Phys. B: At. Mol. Opt. Phys [1] Willke B and Kock M 1993 J. Phys. B: At. Mol. Opt. Phys [13] Mende W, Bartschat K and Kock M 1995 J. Phys. B: At. Mol. Opt. Phys [14] Amin N, Mahmood S, Anwar-ul-Haq M, Riaz M and Baig M A 006a Eur. Phys. J. D 37 3 [15] Amin N, Mahmood S, Saleem M, Kalyar M A and Baig M A 006b Eur. Phys. J. D [16] Saleem M, Amin N, Hussain S, Rafiq M, Mahmood S and Baig M A 006a Eur. Phys. J. D

6 0 M. Atif: Measurement of Photoionization Cross [17] Saleem M, Hussain S, Rafiq M and Baig M A 006b Journal of Applied Physics [18] Hussain S, Saleem M and Baig M A 006a Phys. Rev. A [19] Hussain S, Saleem M, Rafiq M and Baig M A 006b Phys. Rev. A [0] Kalyar M A, Rafiq M, Sami-ul-Haq and Baig M A 007b J. Phys. B: Atom. Mol. Opt. Phys [1] Mahmood S, Amin N, Sami-ul-Haq, Shaikh N M, Hussain S and Baig M A 006 J. Phys. B: At. Mol. Opt. Phys [] Baig M A, Mahmood S, Kalyar M A, Rafiq M, Amin N and Haq S U 007 Eur. Phys. J. D 44 9 [3] Rafiq M, Kalyar M A and Baig M A 008 J. Phys. B: At. Mol. Phys [4] Atif M 014 J. opt. biomed. Mat [5] Burkhardt CE, Libbert JL, Xu J, Leventhal JJ, and Kelley JD 1988 Phys. Rev. A Muhammad Atif did his Ph.D from University of Hull, UK in 005 in the field of Laser Spectroscopy with thesis title Fluorescence dynamics studies of a Dye (PDT Photosensitiser). Presently he is working as Associate Professor at Physics and Astronomy Department King Saud University Riyadh Saudi Arabia. Dr Atif has published 14 papers in ISI journals. Impact Factor for his ISI Publications 1.545, Citations (ISI) 706, H-index 16. He is working on two projects one from NPST and two project from Deanship of research College of Science King Saud University. Dr Atif has good command and vast experience in expermental physics. He is co-founder of National Institute of Laser and Optronics, Islamabad Pakistan.