9822
J. Phys. Chem. C 2008, 112, 9822–9827
Isotope-Selective Chemical Vapor Deposition via Vibrational Activation Daniel R. Killelea,† Victoria L. Campbell, Nicholas S. Shuman,‡ and Arthur L. Utz* Tufts UniVersity, Department of Chemistry, 62 Talbot AVenue, Medford, Massachusetts, 02155 ReceiVed: February 12, 2008; ReVised Manuscript ReceiVed: April 8, 2008
A narrow bandwidth infrared (IR) laser selectively excites and activates one isotopic variant of a gas phase precursor to permit isotope-selective chemical vapor deposition. We show that selective vibrational excitation of the 12CH4 or 13CH4 isotopologue of methane controls the carbon isotope ratio of adsorbates deposited on a Ni substrate and permits significant isotope enrichment in a single reaction step. A model of the process based on known reaction probabilities and process variables predicts the enrichment we observe. While our current laser system permits a 9-fold enhancement of 12C or 13C deposition, optimization of deposition and detection strategies and use of a more powerful, commercially available laser source suggest isotope enrichment factors of 100-fold or more are readily attainable. Other gas-surface reactions activated by vibrational energy may also be candidates for this approach to isotope-selective deposition. Introduction Isotopically enriched reagents have found wide use as mechanistic probes and markers in chemistry for many decades,1 and isotopically engineered materials, nanostructures, and superlattices exhibit unique thermal, electronic, and spin properties that may prove useful in new technologies.2–6 Isotopic composition dictates a bulk solid’s phonon (vibrational) structure and, therefore, many of its physical properties. For example, when the 12C content of a diamond film is increased from 98.89% (natural abundance) to 99.95%, its thermal conductivity is increased 40% at room temperature and 10-fold at lower temperatures.7 Isotopic composition also influences electronic properties, including a semiconductor’s band gap, via electronphonon coupling. Isotope-dependent nuclear spins influence spin coupling and dynamics in solids. Isotopically engineered materials are often fabricated via chemical vapor deposition (CVD) of isotopically purified precursors, but the high cost of purified precursors and the resulting need to recycle the precursor in a CVD or molecular beam epitaxy (MBE) system has limited their widespread use. Preparing isotopically pure reagents and materials is a timeconsuming and capital-intensive process. Current industrial methods exploit small, mass-dependent differences in physical properties to separate compounds containing the isotope of interest. A boiling point difference permits the cryogenic distillation of 13CO from 12CO,8 and UF6 centrifugation to enrich uranium9 relies on mass-dependent mobility. Since most isotopologues have nearly identical masses, their physical properties differ minimally, and separation based on these differences is incremental at best. Significant enrichment is achieved only after many iterations of the separation process. Laser-based methods offer an alternative to physical separation processes. Light resonant with one isotope of a precursor molecule selectively activates the dissociation or reaction of that species, but does not promote reaction in other isotopologues. Early isotope enrichment efforts focused on electronic excitation * Corresponding author. E-mail:
[email protected]. † Current address: James Franck Institute, The University of Chicago, Chicago, IL 60637. ‡ Current address: Department of Chemistry, The University of North Carolina at Chapel Hill, Chapel Hill, NC 27599.
to predissociative states10,11 or infrared multiphoton dissociation (IRMPD) of halomethanes.12–14 These methods resulted in significant enrichment, but they also changed the chemical identity of the targeted isotopologues. When multiple process iterations are required to meet isotopic purity specifications, the chemically altered species must be transformed back into starting materials, which significantly increases process complexity. Recent advances in laser technology and improved chemical methods have led to more economically viable strategies for laser-based isotope separation of carbon15–19 and silicon 20–22 from gas-phase precursors, but these methods are not yet commercially used for bulk production of pure isotopes. In this article, we exploit the recently reported ability of infrared (IR) laser excitation to increase the dissociative chemisorption of methane by a factor of 1000 or more on a nickel surface. 23–25 This observation suggests that isotopeselective vibrational excitation may be an alternative strategy for controlling the isotopic identity of surface adsorbates deposited from gas-phase precursors. Resonant IR excitation of the C-H stretching vibration selectively activates one isotope in an isotopically mixed molecular beam of methane precursor molecules. The vibrationally activated molecules dissociate with much higher probability on the surface, leading to significant isotope enrichment in a single-step chemical process. We model the process, using known reaction probabilities and reagent fluxes, and predict the enrichment we observe. An examination of this model and a consideration of current experimental limitations suggest that a more powerful commercially available laser coupled with an alternative isotope-selective detection strategy would permit much higher levels of isotope purity. We note that this approach produces adsorbate layers with high isotope purity without the need for isotopically purified precursor gases. It has the potential to be scaled up to higher gas fluxes and substrate areas, and could be extended to other gas-surface reaction systems that are strongly activated by vibrational energy. Experimental Details IR laser excitation of methane molecules in a supersonic molecular beam vibrationally activates the dissociative chemisorption of methane on a Ni substrate. Tuning the laser into
10.1021/jp801255q CCC: $40.75 2008 American Chemical Society Published on Web 06/06/2008
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J. Phys. Chem. C, Vol. 112, No. 26, 2008 9823
TABLE 1: Experimental Parameters and Measured Reaction Probabilities for Tnozzle ) 600 Ka 12
Etrans, kJ/mol transition, cm-1 SLaserOn 0 SLaserOff 0 fexc Sν03 (ref 23)
CH4 (98.9%)
48.7 3038.495 4 × 10-5 5 × 10-6 0.096 (8.9 ( 2.0) × 10-4
13
CH4 (1.1%)
51.6 3028.852 7 × 10-5 1 × 10-5 0.096 (1.3 ( 0.2) × 10-3
a The reaction probabilities are the averaged results of replicate measurements. Error limits ((2σ) for SLaserOn - SLaserOff are ( 25%. 0 0
resonance with the ν3 antisymmetric C-H stretch fundamental of either 12CH4 or 13CH4 selectively activates a single methane isotopologue. We then detect the surface-bound methyl products of methane’s dissociative chemisorption with an isotopeselective detection method to assess the isotopic purity of the deposited products. We describe the key features of our apparatus and methods here and list important experimental parameters in Table 1. Prior publications26,27 and the Supporting Information provide additional details. Reagent Preparation. Helium containing 1% CH4 (natural isotopic abundance) expanded continuously from a nozzle held at 600 K (Tnozzle ) 600 K) and formed a triply differentially pumped continuous supersonic molecular beam. Time-of-flight measurements showed that the 12CH4 and 13CH4 isotopologues had the same velocity in the molecular beam, but the 13CH4 isomer had a slightly higher translational energy (Etrans) due to its greater mass. The beam entered an ultrahigh vacuum (UHV) chamber that housed an Auger electron spectrometer (AES), a quadrupole mass spectrometer (QMS), and a Ni(111) sample. A continuous-wave IR laser selectively excited either 12CH4 or 13CH to V ) 1 of the antisymmetric C-H stretching state (ν ) 4 3 via the R(1) transition, 28 and the molecules arrived at the surface in their initially prepared state. Rotational state populations and laser power limited the peak excitation fraction to 9.6% for either isotopologue. The incident methane molecules dissociatively chemisorbed on Ni(111) via cleavage of a single C-H bond to form surface-bound H and CH3 fragments, or they scattered nonreactively from the surface.29 Therefore, only dissociative chemisorption products accumulated on the surface. Isotope-Selective Detection of Reaction Products. Auger electron spectroscopy is unable to distinguish among the isotopes of carbon deposited on a nickel surface, so we needed an alternative method for measuring the carbon isotope ratio of surface adsorbates. Temperature-programmed desorption (TPD) of methane resulting from the recombinative desorption of surface-bound CH3 and H would fulfill this requirement, but methyl dehydrogenates prior to reaction, and no methane desorbs.29 To overcome the lack of reactivity of surface-bound H toward methyl hydrogenation, we used a TPD method developed by Johnson et al.30 They showed that an excess of H atoms deposited into interstitial (subsurface) sites in the nickel lattice reacted quantitatively with surface-bound methyl during a TPD experiment and recombinatively desorbed as methane. A QMS detected and quantified the 12CH4 and 13CH4 products of this H-atom titration. The presence of subsurface H does not alter methane’s reaction probability on the surface.30 The detection scheme required several steps of substrate preparation prior to the CH4 dose and TPD product detection. We exposed a clean Ni(111) surface at 120 K to atomic H, which embedded H atoms into interstitial lattice sites and covered the surface with H. We then cooled the surface to 90 K and exposed the sample to a beam of high-Etrans Xe atoms.
Figure 1. TPD data show H2 evolution and the methane titration products of surface-bound 12CH3 and 13CH3 following a CH4 dose without laser excitation. The traces are offset along the vertical axis for clarity.
The Xe selectively removed surface-bound H by collisioninduced recombinative desorption (CIRD), but preserved H in subsurface lattice sites and regenerated a clean Ni(111) surface for methane dissociation. After the Xe exposure was complete, we introduced the CH4 beam of interest. At 90 K, the nascent methyl products of dissociative chemisorption were stable, but molecularly adsorbed CH4 desorbed promptly. After the methane dose was complete, we applied a 3 K/s temperature ramp to the surface. Subsurface H migrated to the surface and hydrogenated the surface-bound methyls during this TPD experiment, leading to desorption of 12CH4 (m/z ) 16) and 13CH4 (m/z ) 17) near 180 K. The integrated methane TPD peak areas revealed the isotope ratio of reaction products, and AES measurements related the TPD peak area to absolute coverage. For these studies, the reactivity difference with and without laser excitation (SLaserOn - SLaserOff ), divided by the excitation fraction 0 0 (fexc), yielded the inherent reactivity of the ν3 state, Sν03.31 Results Figure 1 shows TPD data for an experiment without laser excitation of the ν3 vibration. The CH4 exposure corresponded to 5400 CH4/surface Ni atom. The resulting methyl coverage was 0.026 monolayer (ML), with ∼97% desorbing as 12CH4 and 3% (0.0008 ML) desorbing as 13CH4. The resulting 13C/ 12C ratio was 0.03 ( 0.01. The 13CH yield exceeded its 1.1% 4 natural abundance because of its slightly higher Etrans in the seeded molecular beam. Methane’s reaction probability increases exponentially with Etrans, and our Etrans-dependent measurements of Sν03 on Ni(111) quantitatively predict the observed enhancement in 13CH4 yield. 23 Next, we used our laser to excite 12CH4, and we quantified the isotopic identity of surface-bound reaction products. Because laser excitation increased reactivity of the majority 12CH4 isotopologue, we reduced the dose to 1800 CH4/Ni. Raw data for one such measurement appears in Figure 2. In that experiment, we detected 0.09 ( 0.01 ML of 12CH3 on the surface, which corresponds to a (9 ( 1)-fold increase in 12CH3 yield relative to an experiment without laser excitation. This enhancement is consistent with a predicted enhancement factor of 7 ( 3 based on (SLaserOn - SLaserOff ) values in Table 1. The 0 0 13C yield was just at the 0.0003 ML detection limit and did not change upon laser excitation. Therefore, vibrational excitation
9824 J. Phys. Chem. C, Vol. 112, No. 26, 2008
Figure 2. TPD following ν3 12CH4 excitation (solid lines). 12CH3 deposition increased 9-fold, but the 13CH3 yield was near our detection limit and did not change upon laser excitation. The blue dashed line reveals 12CH3 yield for an equivalent methane exposure without laser excitation.
Figure 3. TPD data following excitation of 13CH4 to ν3 (V ) 1) (solid lines). The dashed lines show data for the same dose conditions, but without laser excitation. Deposition of 12CH3 (blue) is unchanged by laser excitation of 13CH4. In this experiment, 13CH3 (red) deposition increased 9-fold.
of 12CH4 in the beam increased the 12CH3 product yield from 97% to at least 99.7%, and the 13C/12C ratio decreased to 0.003 ( 0.002. We note that this enrichment occurred even though the 12CH4 excitation fraction was only 9.6%. In Figure 3, we excited 13CH4 to V ) 1 of ν3 prior to detecting the reaction products. The beam exposure corresponded to 5400 CH4/Ni, and we excited 9.6% of the 13CH4 in the beam. The 12CH yield did not change relative to the laser-off experiment, 3 but the yield of 13CH3 increased (9 ( 1)-fold to account for 22% of the total product yield, with a 13C/12C ratio of 0.28 ( 0.03. Discussion Our isotope-resolved product yield data establish that a single quantum of C-H stretch vibration and a modest excitation fraction can selectively enhance one isotope’s deposition rate by nearly an order of magnitude. This result is noteworthy, but closer examination of the factors that influence isotope selectivity suggests that our laser system and restrictions imposed by our isotope-selective detection scheme severely limit the enrich-
Killelea et al. ment we detect. In the sections that follow, we develop a quantitative model that predicts the isotopic enrichment possible in a laser-based method such as ours. We validate the model with our experimental data, and use it to identify opportunities for process improvement. We focus on the case where the IR laser selectively activates the 13CH4 isotopologue, but analogous expressions hold for laser-enhanced deposition of 12CH4. Identity of Reactive Ensembles. Three populations of molecules in the molecular beam produce the reaction products we detect. The first population consists of those 13CH4 molecules in the beam that absorb IR light and are excited to V ) 1 of the ν3 antisymmetric C-H stretching state. This population is the focus of our isotope enrichment method. Not all molecules absorb IR light, though. The fraction of 13CH4 excited, fexc, depends on the population of molecules in the V′′ ) 0, J′′ ) 1 ground-state of the IR transition and the extent to which that transition is saturated. In our experiments, no more than 9.6% of the 13CH4 molecules absorb light; over 90% of the 13CH4 does not. The 13CH4 molecules not excited by the laser are second reagent population in the beam, and the 12CH4 molecules, none of which are resonant with the laser, are the third population. The molecules that do not absorb light still have an inherent reactivity that depends on Tnozzle. At Tnozzle ) 600 K, excited vibrational states that are thermally populated in the molecular beam nozzle dominate reactivity. This results in a strongly nozzle-temperature-dependent reactivity for the second and third populations. All three populations contribute reaction products to the isotope ratio (13C/12C) that we measure. Differing Reactivity of the 12CH4 and 13CH4 Isotopologues. Table 1 shows reaction probabilities, S0, for the 12CH4 and 13CH4 isotopologues in the molecular beam measured in the limit of low surface coverage. The data show that for a thermally excited ensemble of vibrations emerging from a molecular beam expansion at 600 K, the two methane isotopologues differ by about a factor of 2 in reaction probability. The state-resolved reaction probabilities for ν3 differ by slightly less. Separate timeof-flight measurements reveal that the two methane isotopologues have the same velocity in the beam, but 13CH4 has a higher kinetic energy due to its greater mass. Our extensive measurements of S0 for the ν3 vibrational state as a function of Etrans indicate that the Etrans difference for the isotopologues fully accounts for the reactivity differences we observe.23 It is not surprising that isotopologues have similar reactivity because their vibrational coordinates are nearly identical and their ν3 vibrational quantum differs by less than 0.4% in energy. Nonetheless, the modest reactivity difference we observe must be included in any quantitative model of isotope yield. Isotope-Resolved Yield of Reaction Products in Experiments Without Laser Excitation. Some fraction of methane molecules incident on the surface dissociatively chemisorb and deposit methyl and an H atom on the surface. Our TPD method provides an isotope-resolved measure of the number of methyl reaction products on the surface, (N(12CH3) and N(13CH3)). The areal density of surface methyls (σ(12CH3) and σ(13CH3)), integrated over the entire surface, gives N(12CH3) and N(13CH3). respectively. We next consider the contributions of the three reactive populations to the isotope-resolved product yields of carbon. We first look at the reactivity of molecules in experiments without laser excitation. The flux of methane in the molecular beam is uniform across the surface, as shown by spatially resolved AES measurements that find uniform carbon coverage in the absence of laser excitation. Therefore, N(12CH3) and N(13CH3) are simply the products of their corresponding areal
Isotope-Selective CVD
J. Phys. Chem. C, Vol. 112, No. 26, 2008 9825
density (σ(12CH3) and σ(13CH3)) and surface area. The areal densities for 12CH3 methyls in experiments where the laser is off resonance, σ(12CH3)off, depends on the reaction probability for 12CH4 in the beam, ST0 (12CH4) (where T ) Tnozzle), the flux of 12CH4 molecules per unit area time, F(12CH4), and the dose time, t.
σ(12CH3)off ) ST0 (12CH4) · F(12CH4) · t
(1)
An analogous expression applies to the laser-off deposition of 13CH methyls: 3
σ(13CH3)off ) ST0 (13CH4) · F(13CH4) · t
(2)
Since the velocities of the two isotopologues are equal, their fluxes differ only by their relative abundance in the beam.
F(13CH4) ) 0.011 · F(12CH4)
(3)
Equations 1–3 permit a prediction of the carbon isotope ratio for experiments without laser excitation, ROff, where the areal density of both methyl isotopologues is constant across surface area A.
ROff )
)
)
)
13
off
12
off
N( CH3)
A · σ(13CH3)off
An examination of fexc(x,y) allows further simplification. If the laser propagates along the x-axis and the molecular beam is centered on the z-axis, then only the x component of molecular velocity leads to a Doppler shifted absorption frequency. Spatially resolved AES measurements confirm that methyl density is constant in the vertical (y) direction but varies with horizontal (x) position. Consequently, fexc(x,y) is independent of y and reduces to fexc(x) for all values of y. For a single laserbeam crossing geometry, or for an aligned multipass cell where all excitation beams are parallel, fexc(x) is Gaussian in form. Spatially resolved AES measurements reveal a 1/e half-width, w, of 2.4 mm under our experimental conditions. Therefore, we can represent fexc(x,y) in terms of the maximum excitation fraction at the center of the inhomogeneously broadened absorption profile, fexc, max () 0.096 in our experiments) and a Gaussian function, G(w,x).
( )
fexc(x, y) ) fexc(x) ) fexc,max exp
-x2 ) fexc,max G(w, x) (6) w2
N(13CH3)Laser ) ∫∫σ(13CH3)Laser(x, y) dx dy
A · σ(12CH3)off ST0 (13CH4) · F(13CH4) · t ST0 (12CH4) · F(12CH4) · t ST0 (12CH4)
(5)
The number of 13CH3 methyls produced by laser-excited molecules and detected by TPD is the laser-enhanced density integrated over the entire surface area.
N( CH3)
0.011 · ST0 (13CH4)
σ(13CH3)Laser(x, y) ) fexc(x, y) · Sν03(13CH4) · F(13CH4) · t
(4)
The values of ST0(12CH4) and ST0(13CH4) in Table 1 predict a value of ROff ) 0.022 ( 0.008, which is consistent with our experimentally measured value of 0.03 ( 0.01 for a laser-off experiment with Tnozzle ) 600 K. Impact of Spatially Resolved Deposition of Laser Excited Molecules. Homogeneous and inhomogeneous broadening mechanisms influence methane’s excitation in the molecular beam, and lead to nonuniform deposition of laser-excited molecules across the Ni(111) sample. The laser intersects the molecular beam perpendicular to its flow axis, so CH4 molecules in the gently diverging molecular beam have a component of their velocity along the laser’s propagation direction. This transverse velocity results in Doppler detuning for molecules moving toward the crystal edges when the laser is tuned to the center of the inhomogeneously broadened absorption profile. In our geometry, those molecules traveling toward the crystal edge do not absorb IR light. In contrast, those molecules moving along the beam axis and toward crystal center are preferentially excited and experience the maximum excitation probability. As a result, excitation fraction, fexc, is not constant in our laser excitation experiments, but it varies as a function of position across the surface. Prior work from our laboratory allows us to quantitatively account for this effect.32 The spatially resolved surface density of 13CH3 methyls resulting from laser-excited 13CH4 is given by eq 5, where fexc(x,y) is the spatially resolved fraction of 13CH4 molecules excited to the ν3 vibrational state and incident on the surface at coordinates (x,y).
(7)
We define (x ) 0, y ) 0) as the crystal center and ensure that the deposition pattern is centered on our circular crystal. The Ni(111) crystal’s dimensions define the integration limits, and integration over y simply yields the vertical dimension of the crystal at a particular value of x. Therefore, we can rewrite N(13CH3)Laser as a function of x and the radial dimension of our crystal (R ) 5.0 mm).
N(13CH3)Laser ) Sν03(13CH4) · F(13CH4) · t ·
∫2 ·
(R2 - x2)1⁄2fexc(x) dx (8) Isotope-Resolved Contributions to the Product Yield. The laser does not excite all molecules in the beam. Those 13CH4 molecules that do not absorb IR light can still react on the surface to produce 13CH3 methyls. We denote this contribution as background (Bkgd) reactivity. Although laser excitation depletes the V ) 0 vibrational ground-state population by fexc, vibrationally excited molecules that are thermally populated at the source nozzle temperature dominate reactivity under our experimental conditions. Therefore, the reduced V ) 0 population in a laser-on experiment has negligible effect on the ensemble-averaged ST0(13CH4). The density and number of 13CH3 adsorbates from background reactivity in the beam is therefore well approximated by the laser-off reactivity.
σ(13CH3)Bkgd ≈ σ(13CH3)off ) ST0 (13CH4) · F(13CH4) · t (9) N(13CH3)Bkgd ) A · σ(13CH3)Bkgd
(10)
The ratio of 13C to 12C deposition products, ROn, detected by TPD in an experiment where we excite 13CH4, incorporates contributions from laser -excited molecules and background reactivity.
9826 J. Phys. Chem. C, Vol. 112, No. 26, 2008
R ) On
)
Killelea et al.
N(13CH3) N(12CH3) N(13CH3)Laser + N(13CH3)Bkgd N(12CH3)off Sν03(13CH4) · F(13CH4) · t ·
∫ 2 · (R2 - x2)1⁄2fexc(x)dx+
A · ST0 (13CH4) · F(13CH4) · t
)
A · ST0 (12CH4) · F(12CH4) · t (11)
To simplify eq 11, we define χ to be a multiplicative constant that relates the state-resolved reaction probability for the laserexcited isotopologue, Sν03(13CH4), to its laser-off reaction probability at the same kinetic energy and Tnozzle ) T.
Sν03(13CH4) ) χST0 (13CH4)
(12)
The state-resolved reaction probability, Sν03(13CH4), is fixed for a particular quantum state and Etrans, but ST0 (13CH4) depends on nozzle temperature. Consequently, χ depends on three factors: the identity of the laser excited state, Tnozzle, and Etrans. The data in Table 1 predict that, for ν3 excitation of 13CH4 in beams with Tnozzle ) 600 K and Etrans ) 50 kJ/mol, χ ) 130 ( 20. We collect terms in eq 11 and use eq 3 to obtain
ROn )
[ ∫ 2 · (R - x )
0.011 · ST0 (13CH4) · χ ·
2
A·
ST0 12CH4
(
2 1⁄2
]
fexc(x) dx + A
) (13)
We use eqs 4 and 6 to obtain an expression that predicts the laser-based enhancement in the isotope ratio relative to a laseroff experiment. This expression includes effects due to incomplete excitation of molecules in the beam and spatial limitations on deposition due to inhomogeneous broadening of the absorption profile. On
R
) R
Off
·
(
fexc,max · χ ·
∫ 2 · (R2 - x2)1⁄2G(w, x) dx A
)
+1
(14) Numerical integration of eq 14 using our experimental parameters of w ) 2.4 mm and R ) 5.0 mm leads to eq 15.
ROn ) ROff · (0.51 · fexc,max · χ + 1)
(15)
Equation 15 predicts ) 0.23 ( 0.02 for excitation, reasonably close to our experimentally measured value of 0.28 ( 0.03. We use analogous expressions to calculate ROn following laser excitation of ν3 in 12CH4 and find values between 0.003 and 0.004, which agree with the 0.003 ( 0.002 we measure. In the limit where w is much larger than R, all incident molecules are excited with probability fexc, max and eq 15 simplifies further. ROn
13CH 4
ROn ) ROff · (fexc,max · χ + 1)
(16)
Current Limitations and Opportunities for Process Improvement. We first consider restrictions arising from our detection scheme. Our use of subsurface H to titrate surface methyls is sensitive, and it provides the isotopic discrimination we need, but it requires surface temperatures (Ts) below 150 K, the temperature at which bulk H begins migrating to the
surface and recombinatively desorbing. This detection scheme precludes our use of more translationally energetic molecular beams that use H2 as a carrier gas. H2 dissociates on Ni(111), but does not desorb at temperatures of 150 K or below, so H would rapidly block all available surface reaction sites for CH4. Helium does not block surface sites, but its higher mass relative to H2 mandates use of higher nozzle temperatures (Tnozzle) to access kinetic energies where reactivity is measurable. This causes two problems that we now consider. At Etrans ) 50 kJ/mol, raising Tnozzle dramatically increases the thermal population of excited vibrational states in the beam and increases both ST0(12CH4) and ST0(13CH4). This leads to much higher background reactivity for the both isotopologues. We find that when Tnozzle decreases from 600 to 300 K, ST0 (12CH4) at Etrans ) 50 kJ/mol decreases 20-fold. An analogous 20-fold decrease in ST0 (13CH4) would increase χ to 2600 ( 400. Rotational cooling in the beam is not complete at Tnozzle ) 600 K.33 This dilutes population in the V′′ ) 0, J′′ ) 1 groundstate for IR excitation. State-resolved measurements of rotational state populations in our beam indicate that decreasing Tnozzle to 300 K would double fexc,max to 0.19 by doubling the number of molecules in V′′ ) 0, J′′ ) 1. We emphasize that our detection scheme, and not the deposition process, mandates low surface temperatures and a helium carrier gas. Performing deposition at Ts ) 550 K, where surface-bound methyls would dehydrogenate and surface-bound H would rapidly recombine and desorb, would permit use of an H2 carrier gas with Tnozzle ) 300 K. Isotopically enriched C would still accumulate on the surface. The limited power of our IR color center laser (3 mW) also limits isotope enrichment. Newer laser systems with >1 W of output could fully saturate the excitation transition, increasing fexc,max by an additional 50% to 0.28. Additional laser power would also allow us to fully illuminate the entire flux of incident molecules, nearly doubling the spatially averaged excitation fraction from 0.51 fexc,max to 1.00 fexc,max. Taken together, these modifications in the excitation and detection process suggest that there is ample opportunity to improve the values of critical parameters in eq 3. Increasing χ to 2600 ( 400, fexc,max to 0.28, and the fractional spatial coverage of laser excited molecules from 0.51 to 1.00 would permit Ron (13C to 12C) ratios approaching 20 for single-pass excitation of 13CH to ν (V ) 1) in a 1.1% natural abundance methane 4 3 precursor. Other vibrational states are even more reactive than ν3 (V ) 1) and could increase χ further. At Etrans ) 50 kJ/mol, the ν3 overtone, 2ν3, is an order of magnitude more reactive than ν3.34 If excited to saturation, this mode could increase the 13C/12C ratio of surface adsorbates to greater than 200. The high natural abundance of 12CH4 presents an opportunity for even greater isotopic purity. Excitation of 12CH4 coupled with the cited potential improvements could lower the 13C/12C ratio from 0.003 to 5 × 10-6 (5 ppm 13C in 12C). Generality of the Method. We have demonstrated the feasibility of this approach for the methane/nickel system, and suggest that other gas-surface systems that exhibit significant vibrational activation may also be candidates for such control. Our quantitative model of the process highlights several requirements that must be satisfied for successful application of the process. The system must exhibit a high reactivity contrast between the laser-excited-state and the reactivity of thermally excited molecules in the background. Hence, it is not suitable for systems with low activation barriers (including many radical reactions), as there is limited opportunity for laser excitation to increase an already high reaction probability. Second, the
Isotope-Selective CVD addition of vibrational energy to the molecule must increase reactivity well above the thermal background. If statistical reaction theories are applicable, the addition of a chemically significant (relative to the reaction barrier) amount of energy in the form of vibrational quanta will automatically increase reactivity, but dynamical limitations on energy flow in some systems may cause vibrational energy to be more or less effective that statistical predictions.24,27 The process will be most efficient when excitation probability, fexc, is high. Several factors impact this requirement. Candidate molecules should possess a strong IR active vibrational transition capable of depositing a chemically significant amount of internal energy at a wavelength accessible to available laser sources. Excitation fraction will be highest when the molecules are concentrated into a small number of initial quantum states. Taken together, these requirements point to diatomic and small polyatomic molecules as the most appropriate precursors and the excitation of fundamentals or low overtones of high-frequency vibrations, including hydride stretches. The laser-excited molecule must reach the surface without significant collision-induced vibrational energy exchange among the precursor’s isotopologues. The requirements for high excitation fraction and minimal energy scrambling are satisfied by molecular beam methods. Systems that do not require the hyperthermal kinetic energy of a supersonic beam to activate reaction could still be candidates for this approach through careful management of the excitation geometry and the mean free path of the excited reagents. Finally, the practical application of this technique will likely require scale up to larger substrate areas and higher reactive gas fluxes. Lasers that produce several watts of narrow bandwidth IR light are already commercially available. The higher power of these lasers will permit excitation of beams that expose much larger surface areas, as we note above. Deposition rate depends on a variety of factors, including incident reagent flux, the reactivity of the reagents in the beam, and surface reaction kinetics. While the feasibility of the approach will ultimately depend on the reaction rates and extent of activation possible for a specific chemical system, we note that the significantly higher reactivity of laser excited gas-phase precursors in properly chosen chemical systems could well offset the less than complete laser excitation of precursors in the beam. Therefore, deposition rates approaching those of other MBE systems may be attainable. Conclusion Isotope-selective excitation of the ν3 C-H stretching vibration in 13CH4 or 12CH4 with Etrans ≈ 50 kJ/mol enhances deposition of the laser-excited carbon isotope 9-fold. We model the excitation and deposition process to obtain expressions that quantitatively predict the extent of isotope enrichment as a function of known experimental variables. We find that limitations imposed by our detection scheme (and not the deposition process itself) limit the enhancement we observe. On the basis of known reaction probabilities and state populations in the beam, we predict that much higher enrichments are possible with straightforward modifications to the beam conditions. We estimate that such improvements could yield deposited 13C purities greater than 99%, or a 12C adsorbate layer with
