The sticking coefficient of barium on a MgO substrate measured by laser inducedfluorescenceC. Gabbanini, S. Gozzini, and A. Lucchesini Citation: Applied Physics Letters 67, 715 (1995); doi: 10.1063/1.115284 View online: http://dx.doi.org/10.1063/1.115284 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/67/5?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Measurement of nonlinear frequency shift coefficient in spin-torque oscillators based on MgO tunnel junctions Appl. Phys. Lett. 95, 022507 (2009); 10.1063/1.3176939 Work function of MgO single crystals from ion-induced secondary electron emission coefficient J. Appl. Phys. 94, 764 (2003); 10.1063/1.1581376 Growth and characterization of thick oriented barium hexaferrite films on MgO (111) substrates Appl. Phys. Lett. 76, 3612 (2000); 10.1063/1.126723 Investigation of barium titanate thin films on MgO substrates by secondharmonic generation J. Appl. Phys. 76, 1169 (1994); 10.1063/1.357841 Measurement of barium loss from a fluorescent lamp electrode by laserinduced fluorescence J. Appl. Phys. 65, 4595 (1989); 10.1063/1.343255

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The sticking coefficient of barium on a MgO substrate measured by laserinduced fluorescence

C. Gabbanini,a) S. Gozzini, and A. LucchesiniIstituto di Fisica Atomica e Molecolare del C.N.R., Via del Giardino,7 56127 Pisa, Italy

~Received 28 February 1995; accepted for publication 19 May 1995!

We present the measurement of the sticking coefficient of Ba atoms on a MgO substrate by resonantlaser induced fluorescence. The substrate is used as a target for a thermal atomic beam of bariumwhile a laser crosses the substrate and counterpropagates to the atomic beam monitoring its velocityprofile. A signal corresponding to negative velocity atoms with respect to the beam direction allowsto determine the sticking coefficient. It resulted equal to 0.7660.05. © 1995 American Institute ofPhysics.

The knowledge of atomic sticking coefficients is impor-tant for the physics of surfaces and in particular for the depo-sition of materials like superconductive~high Tc) films. At-oms may be adsorbed and successively desorbed from thesurface or may be chemisorbed on the surface. While the firstprocess is generally indicated as ‘‘trapping,’’ the second isknown as ‘‘sticking.’’1 The sticking coefficient indicates therelative amount of atoms that is chemisorbed on the surface.The experimental methods generally used for the stickingcoefficient measurements rely in a postdeposition analysislike Rutherford backscattering spectroscopy~RBS!spectrometry.2 On the other side, very accuratein situ diag-nostics of the deposition processes have been performed withvarious detection systems like mass spectroscopy,3 absorp-tion spectroscopy4 and laser induced fluorescence~LIF!.5

LIF experiments have been reported with temporal6,7 andspatial8,9 resolution. These studies give much information onthe deposition process but the atomic sticking coefficientsare difficult to extrapolate. In fact the analysis is complicatedby the presence of various neutral and ionic molecules andclusters containing the atomic species that can dissociate orrecombine.

In this letter we present an experiment that we have per-formed to measure the sticking coefficient of barium atomson a MgO substrate. The measurement method, based onlaser induced fluorescence in high vacuum conditions, allowsthe determination of the sticking coefficient value in realtime.

The experimental apparatus is shown in Fig. 1. A ther-mal atomic beam of barium is produced in a double vacuumchamber with separate pumps. A skimmer divides the firstchamber equipped with an oil diffusion pump and containingthe oven that is heated to 800–850 K from the second cham-ber where a turbomolecular pump keeps the vacuum below1027 Torr. The estimated flux is between 1012 and 1013 at-oms cm22 s21. The surface is positioned at 1.5 m from theoven and is mounted to be rotated and positioned in or out ofthe atomic beam. A continuous wavelength ring dye laser~Coherent 699-21! pumped by an Ar1 laser and operatingwith a Rhodamine 560 dye is used to excite Ba atoms. Thesingle mode laser (DnL,1 MHz! is tuned to the1S0→1P1

transition at 5535 Å and can be scanned over some GHzaround this wavelength; the frequency scans are controlledby a 7.5 GHz free spectral range spectrum analyzer. Thelaser beam is first sent perpendicularly to the atomic beam bya mirror which is moved by a stepping motor. The laser isfrequency tuned to maximize the fluorescence that is col-lected by a lens and focused on a photomultiplier; the signalis amplified and digitally recorded. The induced fluorescenceallows the measurement of the transverse width of the atomicbeam at the longitudinal position where it collides with thesurface; it resulted to be 5.8 mm.

The surface that we used is MgO~100! with a section of10x10 mm and a thickness of 0.5 mm; it has been kept atroom temperature in this measurement. The substrate wasoptically polished on both sides so it trasmits the laser beamwithout strong scattering of light. The surface is rotated atnormal incident angle with respect to the atomic beam, andthe laser is sent counterpropagating to it. A slit defines an

a!carlo@risc.ifam.pi.cnr.it

FIG. 1. Sketch of the experimental apparatus: m5mirror; rm5removablemirror; smm5stepping motor driven mirror; bs5beamsplitter; at5attenuator. The atomic beam comes from a vacuum chamber containingthe Ba oven that is not plotted.

715Appl. Phys. Lett. 67 (5), 31 July 1995 0003-6951/95/67(5)/715/3/$6.00 © 1995 American Institute of Physics

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observation region of few mm near the surface; this avoidsspatial effects due to the beam divergence which affect themeasurement. When the laser is scanned around the reso-nance frequency it selectively excites different atomic veloc-ity classes by the Doppler effect so that the induced fluores-cence monitors the velocity distribution of the atomic beam.We observed that for a typical dye laser intensity of somehundreds of mW the fluorescence could not be detected fromthe background even without the light scattering of the sur-face. The reason is connected to the production of metastableatoms due to the Ba level configuration. The excited1P1

state can in fact decay by an infrared transition to the1D2

that has a lifetime of 242 ms or to the3D state that has alifetime of about 1 min;10 the rise of the metastable popula-tion produces the depletion of the ground state. A strongground state depletion up to 97% has already been observedby Bernhardt.11 To avoid this effect we strongly attenuatedthe laser to an intensity lower than 100mW.

We first recorded the atomic beam velocity profile with-out surface as shown in Fig. 2~a!; this signal is obtained afteraveraging over several laser scans. A recording of the trans-verse laser scan gives the zero velocity marker. The counter-propagating laser scans the atomic distribution~from the

high to the low velocity classes! being the fluorescence in-tensity proportional to the beam profile:

F~v !52~m/2kT!2v3e2mv2/2kT. ~1!

The atomic distribution maximum corresponds to a velocityof 380 m/s. The recorded signal is essentially due to the moreabundant isotope138Ba with minor unresolved contributionsof the other isotopes, whose shifts are between 127 and 258MHz.12 We recorded in the same way the signal in presenceof the surface, as shown in Fig. 2~b!. Besides the incomingbeam profile, there is now a signal at negative velocitiescorresponding to atoms that are reflected back by the surface.This means that the sticking coefficientS is lower than one;it can be simply calculated by the expression:

S512A2 /A1 , ~2!

whereA2 andA1 are the integrals of the fluorescence in-tensities of negative and positive velocity atoms with respectto the beam direction:

A25E2`

0

I ~v !dv, ~3!

A15E0

`

I ~v !dv. ~4!

In this way there is a direct determination ofS in real time.As the probe laser is continuous, it detects both atoms thatare directly backscattered and atoms that are desorbed afterbeing adsorbed on the surface. Although the two processescannot be separated, the measurement gives the real value ofS. We measuredS for various MgO substrates and, sinceSmay depend on the surface coverage,1 for a long expositiontime of some hours at constant flux. We did not find anyappreciable variation within the experimental error. The mea-sured value resulted inS50.7660.05. The reported error in-dicates the fluctuation ofS in different substrates.

The velocity distribution of backscattered atoms has ashape similar to that of the incoming beam, but it is spec-trally narrower. Its maximum corresponds to a velocity ofabout 200 m/s indicating that a significant part of the atomic

FIG. 3. Normalized atomic beam profile as a function of velocity withseparate best fit curves for positive and negative velocity atoms.

FIG. 2. Atomic beam profile as a function of laser detuningDnL recorded inabsence~a! and in presence~b! of the surface. The laser scans from positiveto negative velocities with respect to the atomic beam direction.

716 Appl. Phys. Lett., Vol. 67, No. 5, 31 July 1995 Gabbanini, Gozzini, and Lucchesini

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kinetic energy is lost in the collision with the surface. As thevelocity distribution of the reflected atoms has a beamlikeshape, the process of direct backscattering should be themain channel of negative velocity atoms; in fact anadsorption–desorption process would cause a Maxwell–Boltzmann distribution. The direct scattering is favored bythe normal incidence angle, while a shallow incidence anglewould favour the adsorption–desorption process. It has beenshown13 that in an inelastic process where trapping can beneglected, the scattered atom distribution does not modifyshape but changes its mean kinetic energy^Ee& following theempirical relationship:

^Ee&5a^Ei&1b2kTS, ~5!

where^Ei& is the mean incident energy, 2kTS is the averageatomic kinetic energy of a gas in equilibrium with a surfaceat temperatureTS , anda, b are two parameters not depend-ing on^Ei& andTS . As shown in Fig. 3, the incoming atomicdistribution is fitted by the beam profile Eq.~1! with T5806K, while the backscattered distribution is well fit by scalingkinetic energy by Eq.~5! after appropriate normalization, forinstance witha50.28 andb50. These values are not signifi-cant as other pairs of parameters in Eq.~5! fit the distribu-tion; a andb can be univocally determined by varyingTS .

The authors wish to thank M. Badalassi and M. Taglia-ferri for technical support.

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717Appl. Phys. Lett., Vol. 67, No. 5, 31 July 1995 Gabbanini, Gozzini, and Lucchesini

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