J. Phys. Chem. 1995,99, 11715-11721
11715
Molecular NO Desorption from Crystalline Sodium Nitrate by Resonant Excitation of the NO3- n-* Transition Richard A. Bradley, Jr.J Eric Lanzendorf,' Maureen I. McCarthy, Thomas M. Orlando, and Wayne P. Hess* Environmental Molecular Science Laboratory, Pacific Northwest Laboratory, Richland, Washington 99352 Received: January 3, 1995; In Final Form: May 15, 1995@
-
We have studied the laser desorption of NO from single crystal sodium nitrate following pulsed 213-nm excitation of the x* 3r2 absorption band localized on the nitrate anion. The excitation laser flux is maintained at low levels ( < 2 MW/cm2) to obtain product distributions free of secondary interactions following fragment ejection from the crystal surface. At low fluence, the NO photodesorption yield is found to be linear with desorption laser power indicating that single-photon absorption events lead to fragment ejection. The desorption yield is enhanced by roughly a factor of 1000 for resonant excitation (213 nm) over nonresonant excitation (266 nm) on a per-photon basis. We determine the relative vibrational, rotational, and translational energy distributions of the neutral NO photoproducts. Significant population in vibrational levels up to v" = 4 is observed and translational distributions for the vrr= 0-3 levels are determined. Rotational state populations and translational energy distributions are well characterized by thermal distributions at the substrate temperature. A local excitation mechanism for NO desorption following resonant excitation is proposed. Under these experimetnal conditions the resonant desorption process (213 nm) is dominated by the photochemistry of the surface nitrate ions. A model for the absorptioddissociation mechanism is proposed that differs from that reported for gas phase ions in that it accounts for the stabilization of the ions due to the crystalline field. The role of exciton migration following resonant excitation is also discussed.
Introduction Laser ablation is an important technique in an increasing number of fields including chemistry, physics, materials science, microelectronics, biology, and medicine.' The utility of laser ablation is derived from the diversity of materials that are amenable to the technique. Nearly any solid material can be vaporized to form concentrated pulsed plumes that can be analyzed by mass spectrometry, laser-induced fluorescence, and other techniques. The combination of laser ablation and mass spectrometry has led to the mass determination of a variety of refractory materials including high molecular weight biomolecule^.^^^ Laser ablation combined with mass spectrometry (LAMS) is being developed as a diagnostic for analysis of atomic and molecular species in mixed hazardous waste^.^ By use of this approach, analysis of complex, multicomponent mixtures can be performed rapidly using very little sample. Reduced sample size is highly desirable for the analysis of many hazardous wastes because it minimizes the secondary waste generated from the analytic procedures. In addition to the minimal sample requirements, the LAMS approach reduces sample preprocessing and allows for the handling of thermally unstable samples. The LAMS technique is being developed to analyze mixed wastes (radionuclides chemical mixtures) extracted from the underground storage tanks at the Hanford nuclear reservation. A major component of these tanks is sodium nitrate (NaNO3); hence, understanding the desorption mechanisms of this material is essential to analyzing these wastes.
+
* Author to whom correspondence should be addressed.
' Associated
Westem Universities Postdoctoral Fellow. Permanent address: Optical Coatings Laboratory, Inc., 2789 Northpoint Parkway, Santa Rosa, CA 95407-7397. Department of Chemistry, University of Califomia, San Diego, La Jolla, CA 92093. Abstract published in Advance ACS Abstracts, June 15, 1995. @
0022-3654/95/2099-11715$09.00/0
Knowledge of the desorption mechanisms is needed to determine the correlation between the desorption products and the original sample composition. Under certain conditions (e.g., high laser fluence) the final products may be heavily influenced by the chemistry occumng in the laser-induced plasma near the surface. A detailed description of the photochemical mechanisms is less crucial when LAMS is used solely as a tool for elemental analysis where only the percent composition, and not the molecular speciation, of a material is sought. However, it is critically important to understand the chemistry of the desorption process if LAMS is to be used as a diagnostic tool for determining the chemical composition and structure of the original material. The present work examines conditions that permit laser desorption to be used as a chemical analytic probe. The technique of laser ablation is practiced under a wide range of experimental conditions. Excitation wavelengths can vary from ultraviolet to infrared and pulse durations from femtoseconds to microseconds. Peak pulse powers can also vary over many orders of magnitude inducing a range of physical processes dependent upon laser pulse power. The mechanisms for coupling of the laser energy into the solid itself, including linear absorption, thermal and optical runaway, and electronneutral and electron-ion inverse Bremsstrahlung: are very dependent upon laser intensities. After vaporization, the subsequent reactions of electrons, ions, and neutral molecules can easily alter the chemical composition of the vaporized material. The role of many of these secondary reactions can be drastically reduced by maintaining a low number density of desorbed species by operating in a controlled low laser fluence regime. High particle fluences can result in Knudsen layer formation which may dramatically alter particle velocities and internal energy distribution^.^ In a recent article,6 Dreyfus defined the high and low irradiance regimes to be above 10" W/cm2 and below lo9 W/cm2, respectively. In the present work, we explore the "very 0 1995 American Chemical Society
Bradley et al.
11716 J. Phys. Chem., Vol. 99, No. 30, 1995 low" irradiance regime using powers of -lo6 W/cm2, 3 orders of magnitude lower than the low irradiance regime defined by Dreyfus. Experiments using very low irradiance are possible due to the high efficiency of W laser desorption and the sensitivity of laser ionization techniques under ultrahigh vacuum (UHV)conditions. Under low irradiance conditions, the emitted particle number density is sufficiently low for the molecules to disperse without collisions allowing for the determination of nascent product state distributions of desorbed molecules. The laser-surface-interaction literature favors the word desorption to describe processes that meet these conditions and reserves the word ablation for conditions of high surface removal rates on the order of a monolayer per pulse or greater. Laser desorption of many simple (MX) ionic crystals has been reported by several groups, with the alkyl halides receiving particular a t t e n t i ~ n . ~ There - ~ has also been extensive study of alkyl halide sputtering by the related technique of electronstimulated desorption (ESD).l0-l5 Laser desorption studies of alkaline earth ionic crystals have also probed the excitation mechanism and the dynamics of neutral and ionic product formation.16-18 Less work has been directed at probing the desorption mechanisms in molecular ionic crystals such as NaNO3 or N&N03.19 These crystals are interesting because they possess strong "molecular" absorptions in an accessible W region. The photochemistry of the molecular ion chromophore, in turn, dictates the final product distributions. In this paper we investigate, in detail, the mechanisms and consequences of laser desorption on crystalline sodium nitrate in order to deduce the optimal conditions for applying laser desorption as a "chemical" analytical diagnostic. Sodium nitrate is a wide band gap, insulating material that forms a molecular ionic crystal with a hexagonal unit cell of D3d symmetry. The bulk absorption spectrum displays a strong absorption feature, centered at 6 eV, that has been assigned to a n* n2 transition localized on the nitrate ion.20 The initial bulk absorption into the n* state leads to the formation of an exciton localized on the nitrate ion latttice site. A second strong absorption feature centered at 10.5 eV has been assigned as a charge transfer transition moving electronic density from the nitrate ion to the empty Na+ (3s) orbital. A forbidden transition (1000 times weaker) assigned to the n* n2 transition peaks at 4.3 eV. In this paper we focus on laser desorption following photoexcitation of the 6-eV band and contrast the results with preresonant excitation at 4.66 eV. The desorption mechanisms are deduced from the experimental data in conjunction with theoretical ab initio quantum mechanical calculations that examine the effects of the crystalline environment on spectroscopy and the ion chemistry.21 We examine laser/solid interactions following resonant excitation of single-crystal sodium nitrate by measuring the translational, rotational, and vibrational energy distributions of desorbed NO. Our results reveal that resonant laser excitation provides much higher NO yields than nonresonant excitation.
-
-
Experimental Section The experimental apparatus consists of a liquid nitrogen trapped diffusion-pumped UHV chamber (base pressure 10-lo Torr) equipped with commercial LEED and Auger optics, a laser tier with laser access viewpoints, and quadrupole (QMS) and time-of-flight (TOR mass spectrometers. The sample manipulator is mounted vertically on an xy micrometer stage with sufficient z-stroke to access the electron spectrometers and the laser tier. The sample manipulator also has the capability to rotate the sample about the z-axis and may be cooled to near liquid nitrogen temperatures (-90 K). Figure 1 is a schematic
Input window for QMS experiments
\
Figure 1. Schematic diagram of the experimental apparatus showing the optical layout of the desorption and ionization lasers, the CCD camera, the quadrupole and TOF mass spectrometers, and the sample holder. Not shown are Auger and LEED surface diagnostics, which are located on a separate tier.
of the laser tier illustrating the location of the QMS and TOF mass spectrometers as well as the optical geometry for the various ionization detection schemes used. Experiments were performed using both melt- and solutiongrown single crystals of NaNO3. Sodium nitrate forms a hexagonal lattice (space group 167, R j c ) with a rhombohedral primitive cell. Melt-grown crystals are obtained from Bicron, and solution-grown NaNO3 samples were formed by slow evaporation from saturated aqueous solutions over 2-3 week periods to produce large (-1 cm3) clear crystals. The solutiongrown crystals were oven dried at 160 "C for 16 h to remove excess water22and then were stored under vacuum until usEd. Both types of crystals were cleaved in air along the 1014 cleavage plane of the hexagonal lattice, and the resulting chips were mounted with spring clips on the vacuum manipulator. In UHV, the NaNO3 crystal (mp 308 "C) is heated to 200 "C for 16 h to reduce water contamination of the surface; a similar heating procedure is used in UHV studies of NaCl crystals although samples are heated to higher temperature^.'^,^^ The NaNO3 crystals were excited using 5-ns pulses of 213nm light incident on the sample at 40" to the crystal face. The 213-nm laser was generated by mixing the fundamental and the fourth harmonic output of a 20-Hz Nd:YAG laser in a B-barium borate (BBO) crystal. The resulting 213-nm beam was passed through a spatial filter to form a well-defined Gaussian profile that was monitored by a CCD camera. The desorption laser pulse power was recorded using a photodiode output to a boxcar integrator. For resonant 213-nm excitation, pulse energies of 150 pJ or less, in a 2-mm spot diameter, were directed onto the sample. Under these conditions, the maximum laser irradiance is 1 MW/cm2. The ions directly produced in the desorption process were detected with a Wiley -McClaren type time of flight (TOF) spectrometer with a resolution of &Am = 150 at m/z = 30. Ion signals were captured on a digital scope and stored on a personal computer. Although accurate material removal rates are difficult to determine precisely, best estimates of desorption yields (from QMS and TOF signal intensities and preliminary atomic force microscopic measurements) indicate that less than 1% of a monolayer is removed per laser pulse, consistent with the low laser fluences used. Neutral NO fragments are probed using 1 1 multiphoton
+
NO Desorption from Crystalline Sodium Nitrate ionization (MPI) following desorption. The probe laser system consists of a Nd:YAG-pumped dye laser whose output is frequency tripled by doubling in KDP and mixing the fundamental and second harmonic in BBO; this frequency generation scheme covers the 210-233 nm spectral region for (1 f 1) MPI detection of the v" = 0-4 states of NO. The positive ions produced via MPI are detected using the TOF mass spectrometer, and the relative ion yields are measured using a boxcar integrator. Both lasers are triggered using a variable digital delay generator. The velocity measurements are taken with the delay between the the lasers stepped in 40 ns increments, and typically, 20 shots are averaged. The micro channel plate (MCP) ion detector used in the TOF spectrometer is sensitive to photoelectrons generated by the 213-nm light, resulting in a large scattered light signal. To reduce the scattered light interference, a high voltage pulse (400 V, -200-ns width) is applied to the front plate of the MCP to attenuate the gain for h100 ns with respect to arrival of the 213-nm light pulse. This procedure dramatically reduces the ringing associated with the scattered light signal and diminishes the interference with the measured translational energy distributions. The (l+l) laser ionization scheme used in this study must be implemented carefully to avoid saturation effects that may skew apparent rotational and vibrational state populations. In detailed studies of the (1+1) MPI scheme for NO detection, Jacobs et al.24325showed that saturation of the transition is sensitive to the specific rovibronic transition probed and varies across the different branches and even across specific transition line shapes. Furthermore, intermediate state alignment in the ionization process can also affect the ion intensities but usually to a lesser extent.24 Saturation effects were shown to have up to a 10% effect on the resulting temperature extracted from the data.24925For this reason, in our measurements of NO rotational distributions, we have chosen to focus on the P12 band of each vibronic transition studied and correct only for the power dependence measured at the P12 bandhead. The accuracy of this approach is verified by measuring the rotational state distribution for room temperature NO leaked into the UHV chamber at Torr. The Pl2 band is also selected for investigation due to lack of spectral interference from the other 11 rotational branches. We measure state population distributions using probe lasers powers of
