Search

A new approach to atomic layer etching (ALE) has been demonstrated, and its application to 4H-SiC is reported here. By pulsing only the DC bias for an Ar/Cl 2 inductively coupled plasma-reactive ion etching system, the etch cycle duration is reduced by more than an order of magnitude relative to conventional ALE processes. Gas flows are not changed throughout the ALE process. With this process protocol, we achieved an etch rate of 2.48 ± 0.09 Å/cycle with 6 s cycles, an RMS surface roughness (R q ) of 0.83 ± 0.08 Å, and an ALE synergy value of S = 99%. The parameters explored within this ALE process demonstrate effective subangstrom smoothening of 4H-SiC surfaces and is well-suited for a variety of classical and quantum device nanofabrication. [ABSTRACT FROM AUTHOR]

Coupling between two microcavity plasmas in a symmetric, microfabricated dielectric barrier structure has been observed by injecting charge from one of the plasmas into an intervening microchannel. Periodic modulation of the electric field strength in the injector (or electron "donor") cavity has the effect of deforming the acceptor microplasma which exhibits two distinct and stable spatiotemporal modes. Throughout the time interval in which the two microplasmas are coupled electrostatically, the acceptor plasma is elongated and displaced by 75-100 μm (~30% of its diameter) in the direction of the microchannel. The depletion of charge in the microchannel results in an immediate transition of the second (acceptor) microplasma to an equilibrium state in which the plasma is azimuthally symmetric and centered within its microcavity. Switching between these two spatial modes requires a shift (in the plasma centroid) of ~80 μm in <50 ns which corresponds to a velocity of 1.6 km/s. Precise control of this plasma phase transition through device fabrication, and modulation of the donor plasma electric field, suggest applications of this plasma coupling and charge transport mechanism to signal processing, photonics, and electromagnetics. [ABSTRACT FROM AUTHOR]

The diatomic collisional intermediate responsible for the formation of an electronically excited molecule by teratomic recombination has been observed in both the spectral and temporal domains by laser spectroscopy. We report experiments demonstrating thermal Xe(6s[3/2]2)-Xe(5p6 1S0) atomic collision pairs to be the immediate precursor to the formation of Xe*2(a³Σ+u, A¹Σ+u) by the three body process: Xe*(6s)+2Xe → Xe*2 + Xe, where the asterisk denotes an excited electronic state. Photoassociating Xe(6s)-Xe atomic pairs by free ← free transitions of the collision complex interrupts the production of the electronically excited Xe dimer, thereby suppressing Xe2 spontaneous emission in the vacuum ultraviolet (VUV, λ ~ 172 nm, A¹Σ+u → X¹Σ+g). Intercepting Xe(6s)-Xe pairs before the complex is stabilized by the arrival of the third atom in the teratomic collision process selectively depletes the pair population in a specific Franck-Condon region determined by the probe laser wavelength (λ). Measurements of the variation of VUV emission suppression with λ provide a spectral signature of the [Xe(6s[3/2]2) - Xe(¹S0)]* complex and map the probe laser wavelength onto the thermal energy (∊″) of the incoming collision pairs. [ABSTRACT FROM AUTHOR]

Several Förster resonance energy transfer (FRET) lasers have been realized by employing the robust and versatile streptavidin-biotin (SPB) biocomplex as the acceptor–donor linkage. SPB offers a fixed acceptor–donor separation ("ruler") of <6 nm, which lies within the Förster radius for a broad range of donors and acceptors. A Cy3-SPB-Cy5 conjugate laser (where Cy3 and Cy5 are cyanine dyes) peaking at λ ∼ 708 nm has been observed, and its bandwidth and threshold pump energy (at 532 nm) have been measured to be ∼4.5 nm and 118 µJ (corresponding to a pump energy density of 179 ± 5 µJ/mm2), respectively. Depolarization of the linearly polarized pump optical field by this FRET process is found to be <12%. To tether the acceptor and donor, the SPB complex requires only that either be conjugated, thereby allowing FRET processes to be examined among an extensive set of biomolecules, inorganics, and nanoantenna acceptors, for example. As a result, fluorophore-nanoparticle lasers having characteristics of both FRET lasers and plasmonic emitters have been demonstrated. Laser spectra and the phase shift induced by a 10 or 100 nm gold nanoparticle tethered to the Cy3-SPB complex suggest that both the fluorescent protein and nanoparticle are able to act as an acceptor. The brightness associated with this new class of fluorophore/nanostructure FRET lasers will broaden the scope of accessible biomedical diagnostics, including cellular imaging and the detection of DNA and proteins. [ABSTRACT FROM AUTHOR]

Coupled oscillators are prevalent in nature and fundamental to fields as disparate as astrophysics, photonics, the mechanical sciences, and geophysics. Theory has identified singularities in the response of coupled oscillators, known as exceptional points (EPs), that are associated with non-Hermitian operators and lie at the transition between weak and strong coupling of the oscillator. Although several EPs have been reported or predicted to exist in nanophotonic resonators and Feshbach resonances, for example, tuning the phase of two interfering atomic or molecular coherences near an EP has not been demonstrated previously. We report the observation of an EP associated with a pair of interfering atomic coherences in Rb, oscillating at 386.3 and 384.2 THz, and confirm the theoretical prediction of an abrupt phase shift of ∼π/4 as the EP is traversed by independently varying two experimental parameters. Pairs (and trios) of coupled coherences in thermal Rb atoms are established among the 7s1/2, 5d5/2, 5p3/2, and 5s1/2 states in pump–probe experiments with <200 fs laser pulses, and observed directly in the temporal and spectral domains through the ensuing quantum beating in the ∼2–36 THz interval. Interference between the (5d5/2–5p3/2) and (5p3/2–5s1/2) coherences is mediated by the 5p3/2 state and detected through quantum beating in the vicinity of the (5d5/2–5p3/2)–(5p3/2–5s1/2) difference frequency of 2.11 THz which is monitored by a parametric four-wave mixing process. Phase of this composite atomic oscillator is first controlled by varying the mean Rb–Rb nearest neighbor distance (⟨R⟩) in a thermalized vapor. A discontinuous transition of (0.8 ± 0.2) ∼ π/4 radians in the phase of the coupled oscillator occurs when ⟨R⟩ is varied over the ∼80–90 nm interval, a phase shift associated with the transformation of a broadband, dissipative oscillator (characterized by a Fano interference window) into a strongly-coupled system resonant at 2.1 THz. [ABSTRACT FROM AUTHOR]

Bound←bound transitions of the Xe dimer at small internuclear separation (R < 4.0 Å) have been observed in the 545-555 nm and 675-800 nm spectral regions by laser spectroscopy in the afterglow of a pulsed Xe microplasma with a volume of ∼160 nl. Transient suppression of Xe2 A¹Σu+ (Ou+) → X¹Σg+ (Og+) emission in the vacuum ultraviolet (∼172 nm), induced by laser excitation of Ωg ← a³Σu+ (1u,Ou-) [Rydberg←Rydberg] transitions of the molecule, has confirmed the existence of structure between 720 and 770 nm (reported by Killeen and Eden [J. Chem. Phys. 84, 6048 (1986)]) but also reveals red-degraded vibrational bands extending to wavelengths beyond 800 nm. Spectral simulations based on calculations of Franck-Condon factors for assumed Ωg ← a³Σu+ transitions involving Ω = 0±,1 gerade Rydberg states suggest that the upper level primarily responsible for the observed spectrum is an Ω = 1 state correlated, in the separated atom limit, with Xe(5p6 ¹S0) + Xe(5p5 6p) and built on a predominantly A²Π3/2g molecular ion core. Specifically, the spectroscopic constants for the upper state of the 1g ← 1u,Ou± absorptive transitions are determined to be Te = 13 000 ± 150 cm-1, ωe′ = 120 ± 10 cm-1, ωexe′ = 1.1 ± 0.4 cm-1, De = 3300 ± 300 cm-1, and ΔRe = Re′- Re″ = 0.3 ± 0.1 Å which are in general agreement with the theoretical predictions of the pseudopotential hole-particle formalism, developed by Jonin and Spiegelmann [J. Chem. Phys. 117, 3059 (2002)], for both the (5)1g and (3)Og+ states of Xe2. These spectra exhibit the most extensive vibrational development, and provide evidence for the first molecular coreswitching transition, observed to date for any of the rare gas dimers at small R (<4 Å). Experiments in the green (545-555 nm) also provide improved absorption spectra, relative to data reported in 1986 and 1999, associated with Xe2 Rydberg states derived from the Xe(7p) orbital. [ABSTRACT FROM AUTHOR]

An analytic expression for the electromagnetic enhancement of the spontaneous emission rate and Raman scattering cross-section for an excited atom or molecule in close proximity to a metal nanocylinder has been derived by quantum theory. Coupling of the atomic or molecular optical radiation into the TM0 surface plasmon mode of the nanocylinder results in reradiation by the cylinder, a process that is most efficient when the incident radiation is linearly polarized, with the electric field oriented parallel to the axis of the nanocylinder. For a silver cylinder having a radius and length of 5 and 20 nm, respectively, the enhancement in the spontaneous emission rate is >107 for hω0≃2.4 eV (λ=514 nm), which corresponds to an increase of ≈1014 in the Raman scattering cross section. This result, as well as the prediction that the atomic dipole generates broadband, femtosecond pulses, are in qualitative agreement with previously reported experiments involving metal nanoparticle aggregates. The theoretical results described here are expected to be of value in guiding future nonlinear optical experiments in which carbon nanotubes or metal nanowires with controllable physical and electrical characteristics are patterned onto a substrate and coupled with emitting atoms or molecules. [ABSTRACT FROM AUTHOR]

Two well-developed Rydberg series of atomic iodine, built on the ...5s[sup 2]5p[sup 4] [sup 3]P[sub 1] ion core and assigned to nd[2][sub 5/2] and ns[1][sub 3/2] states with principal quantum numbers as high as n=47, have been observed by two photon (566≤λ≤600 nm) ionization of I(6s [sup 2]P[sub 3/2]), and time-of-flight photoelectron spectroscopy and mass spectrometry. The data reported here extend the nd[2][sub 5/2] and ns[1][sub 3/2] series by 27 and 23 terms, respectively. The limit for both series, 91 387.9±1.3 cm-1, revises upwards by 5.8 cm-1 the value for the I[sup +]([sup 3]P[sub 1]) state energy suggested by Minnhagen in 1962 [Ark. Fys. 21, 415 (1962)]. The quantum defects for the nd[2][sub 5/2] and ns[1][sub 3/2] series were determined to be 2.44±0.03 and 4.03±0.02, respectively. Several terms of a third, weaker series were also detected and tentatively assigned to nd[2][sub 3/2] states, also having the [sup 3]P[sub 1] ion core. Photoelectron peaks associated with direct, five photon ionization of I[sub 2] have been observed and are attributed to electronic autoionization of A [sup 2]Π[sub (3/2)u] ion core Rydberg states into the X [sup 2]Π[sub (1/2)g,(3/2)g] ionization continua. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Inter-Rydberg transitions of Ar2 in the green (∼460–530 nm) have been studied by intracavity, laser absorption spectroscopy. All five Hund’s case (c) members of the Aπu5p manifold, predicted by the guidelines established by Mulliken [J. Chem. Phys. 52, 5170 (1970)] for Xe2 and later adapted by Chang and Setser [J. Chem. Phys. 69, 3885 (1978)] for Ar2, have been observed. The molecular transitions originate from Aσg4s(1u,0-u) and terminate on the A2 Σ+ 1/2 u ion core, 3Πg on the A2 Σ+ 1/2 u ion co-derived (5p Rydberg electron 0-g, 0+g, 2g, 1g, and 1g. Transitions involving three of these are strongly red degraded [revealing that Re4s(1u, 0-u) g-) <=Re(0+g)

Absolute photoabsorption cross sections for the lowest excited state (4 2Γ) of Kr2F and the ground state (1( 1/2 )u) of Kr+2 have been measured in the visible and ultraviolet (Kr2F: 248, 308, 335–600 nm; Kr+2 : 360–500 nm). As predicted by theory, the Kr2F spectrum is dominated by the 9 2Γ←4 2Γ band which peaks below 340 nm and correlates with the Kr+2 2( 1/2 )g ←1( 1/2 )u transition. However, Kr2F(4 2Γ) and Kr+2 (1( 1/2 )u) both absorb more strongly for wavelengths beyond ∼450 nm than expected, apparently owing to a nonthermalized Kr+2 vibrational distribution. Despite interference from Kr2F stimulated emission between ∼350 and 460 nm, the experimental results show the Kr2F(4 2Γ) and Kr+2 absorption profiles to be similar for λ≥460 nm but diverging rapidly as λ increases from 335 to 360 nm. At 360 nm, the absorption cross sections differ by a factor of 3. Contrary to theoretical expectations, the Kr2F absolute absorption cross section is consistently smaller (typically <40%) than that for Kr+2 which suggests that the impact of F- on the Kr+2 oscillator strengths is significant. [ABSTRACT FROM AUTHOR]

The emission spectrum of Br2 in the ultraviolet (180≤λ≤400nm) has been examined in electron beam pumped rare gas–hydrogen bromide (HBr) gas mixtures. Temporally resolved and rare gas pressure-dependent studies of the Br2 fluorescence at 292 (D’→A’), 312, 334, and 358 nm indicate that the formation processes for Br2 ion pair states differ for Ne and Ar diluents. Experimental evidence suggests that Br2 excited states in the first tier [correlated with Br-(1S0)+Br+(3P2)] in Ar/HBr mixtures are formed by ion–ion recombination in which Br+(3P2) is produced by charge transfer between Ar+(2P) and HBr, followed by predissociation of HBr+(A 2Σ+). In Ne buffer, Br2 states well above D’ are apparently produced by Penning ionization of HBr by Ne 3s 3P followed by recombination of Br+(3P0,1,2 ) ions with Br-(1S0). Also, Ar is effective in collisionally relaxing the lowest manifold of six Br2 ion pair states but both Ne and Ar are poor quenchers of the D’(2g) level. [ABSTRACT FROM AUTHOR]

Absolute cross sections for the photoionization of Kr (5s) and Kr2 5s 3Σ+u (1u, 0-u ) at 248 nm have been measured to be (5.7±1.1)×10-19 and (1.4±0.3)×10-18 cm2, respectively. In these experiments, absolute photoelectron densities were measured in real time with a microwave absorption apparatus, and the excited atomic and molecular species were produced and subsequently photoionized by excimer lasers. The measured Kr (5s) cross section is consistent with the upper limit experimentally determined by Dunning and Stebbings [Phys. Rev. A 9, 2378 (1974)] and is close to the theoretical value calculated by Hartquist [J. Phys. B 11, 2101 (1978)]. For Kr2 5s 3Σ+u (1u, 0-u) , the cross section reported here is intermediate to those measured previously for the lowest 3Σ+u states of Ar2 and Xe2. [ABSTRACT FROM AUTHOR]

Absorption from the lowest-lying ion pair state of HgBr (i.e., Hg+–Br-: B 2∑+1/2 ) has been observed by transient absorption spectroscopy in a pulsed, transverse discharge containing a mixture of Ne, N2, and HgBr2 vapor. A highly structured band peaking at ∼447 nm is shown to be associated with charge–transfer transitions from the B 2∑+1/2 state to a higher-lying, covalently bonded (Hg*–Br) state. By pumping the 447 nm transition with a pulsed dye laser while simultaneously monitoring gain on the HgBr (B 2∑+1/2 →X 2∑+1/2 ) band at 502 nm, the absorption cross section for the blue band was measured to be (3.0±1.5)×10-17 cm2 and the saturation intensity for the transition at 447 nm was determined to be 710±50 kW cm-2. [ABSTRACT FROM AUTHOR]

Excitation spectra of the 5s 3Σ+u(1u,0-u) state of the Kr2 molecule have been obtained in the wavelength interval 335≼λ≲545 nm by monitoring atomic Kr emission at 758.7 nm (5p[1/2]0→5s[3/2]1) that accompanies dye laser excitation of the neutral excited molecule. The dominant peaks in the spectrum are identified as arising from np 3Πg←5s 3Σ+u Rydberg transitions where 7≤n≤18. The series limit and quantum defect δ for this series have been determined to be 28 471.9±3.9 cm-1 (351.12±0.05 nm) and 2.629±0.004, respectively, and the vibrational frequency ωe of the 5s 3Σ+u(1u,0-u) excimer has been estimated to be ∼195 cm-1. All of the observed Rydberg states have an A 2Σ+1/2u[1(1/2)u] ion core. Comparison of the np 3Πg←5s 3Σ+u series limit and the ionization potential of Kr* 5s[3/2]2 shows that the A 2Σ+1/2u ground state dimer ion is ∼0.55 eV more deeply bound than is the Kr2 5s 3Σ+u neutral species. Also, dissociation energies of the np 3ΠgRydbergs are shown to be nearly identical to that of the A 2Σ+1/2u dimer ion state. A second series of spectral peaks has also been identified and assigned to n′p 3Σ+g ←5s 3Σ+u transitions. Analysis of the series yielded a limit of 28 424±120 cm-1 and a quantum defect of 3.39±0.04. Two members of a third, unidentified series having a quantum defect of 3.24±0.01 and also converging to A 2Σ+1/2u (v′=0) have been observed. Absolute photoabsorption cross sections for the np 3Πg←5s 3Σ+u (7≤n≤18) transitions have also been determined. [ABSTRACT FROM AUTHOR]

Transient absorption spectra of rare gas dimer Rydberg transitions in the ultraviolet, visible, and near infrared (220≤λ≤900 nm) spectral regions have been observed in electron beam excited rare gases. The most prominent molecular bands for He2, Ne2, Ar2, Kr2, and Xe2 are assigned to the mp 3Πg←ns 3Σ+u (He[ATOTHER]@B:[/ATOTHER]n=2, 3≤m≤10; Ne[ATOTHER]@B:[/ATOTHER]n=3, 4≤m≤10; Ar[ATOTHER]@B:[/ATOTHER]n=4, 5≤m≤15; Kr[ATOTHER]@B:[/ATOTHER]n=5, 6≤m≤16; Xe:n=6, 8≤m≤11) Rydberg series of the dimer. Adiabatic ionization potentials, relative to the ns 3Σ+u state, are determined by extrapolation of the series to their limits (m→∞) to be 34 361.4±30 cm-1 (4.260±0.004 eV) for He2, 34 396.3±25 cm-1 (4.265±0.003 eV) for Ne2, 29 351.9±15 cm-1 (3.639±0.002 eV) for Ar2, 28 428.5±10 cm-1 (3.525±0.001 eV) for Kr2, and 26 664.5±250 cm-1 (3.306±0.031 eV) for Xe2. Absorption bands which are ascribed to m’p 3Σ+g←ns 3Σ+u Rydberg series of Ne2(m’=4,6,8), Ar2(5≤m’≤10), Kr2(6≤m’≤10), and Xe2(m’=8,9) are also reported, and the ns 3Σ+u ionization potentials found to be identical to the respective values quoted above. All of the observed molecular Rydberg states have an A 2Σ+1/2u ion core. The lowest vibrational quanta (ΔG1/2) for the ns 3Σ+u states are determined from vibronic structure in the Rydberg transitions to be 1723 cm-1 for He2, 539 cm-1 for Ne2, 302 cm-1 for Ar2, 172 cm-1 for Kr2, and 135 cm-1 for Xe2, and are consistent with previously reported values. The instability of the ns 3Σ+u metastable state with respect to the dimer ion core potential [D0(A 2Σ+1/2u)-D0(ns 3Σ+u)] is calculated to be 0.508±0.004 eV for He2, 0.681±0.003 eV for Ne2, 0.572±0.002 eV for Ar2, 0... [ABSTRACT FROM AUTHOR]

Rydberg series of Ar2 and Kr2 have been observed in the visible and ultraviolet (350≤λ≤560 nm) absorption spectra of the lowest-lying metastable states of the dimer (ns 3Σ+u(1u, 0-u); Ar: n=4, Kr: n=5). The most prominent series for Ar2 and Kr2 are assigned to ns 3Σ+u → mp 3Πg (Ar: 5≤m≤15; Kr: 6≤m≤16) Rydberg transitions of the diatomic molecule and adiabatic ionization potentials are determined from an extrapolation of the series to their limits (m → ∞) to be 116 356±340 cm-1 (14.43±0.04 eV) for Ar2 and 104 616±150 cm-1 (12.97±0.02 eV) for Kr2. The dissociation energies of the A2Σ+u[1( (1)/(2) )u] dimer ion states of Ar2 and Kr2 are, therefore, 1.33±0.04 and 1.03±0.02 eV, respectively. All of the observed Rydberg states have an A2 Σ+u ion core. Binding energies of the lowest observed 3Πg states, were determined to be (1.331±0.005) eV for Ar2 and (1.030±0.001) eV for Kr2. The quantum defects for the Ar2 and Kr2 series (1.656±0.006 and 2.621±0.004, respectively) and ωe-2ωexe for the ns 3Σ+u metastable state of Ar2 (303±20 cm-1) and Kr2 (172±10 cm-1) were also measured and the vibrational frequencies are consistent with previously reported values. Absorption peaks which are attributed to ns 3Σ+u→m’p 3Σ+g Rydberg transitions of Ar2 (6≤m’≤10) and Kr2 (7≤m’≤10) are also reported. [ABSTRACT FROM AUTHOR]

The absorption cross section for the lowest-lying, bound excited state (4 2Γ) of Xe2Cl has been measured at several wavelengths in the ultraviolet (UV). Temporally isolating the excited Xe2Cl species in Xe/Cl2 gas mixtures has been accomplished by producing XeCl molecules in the B state by photoassociation [Xe+Cl+hω → XeCl(B)] at λ=308 nm. Collisional mixing of the lowest lying ion pair states of the excimer subsequently forms the XeCl species in its C state. Following the formation of Xe2Cl* from XeCl(B,C) by a three-body collision, the long radiative lifetime of the trimer (>200 ns) relative to those for the XeCl B and C levels is exploited to ensure that the triatomic molecule is the predominant species when the experiments are performed. A second UV laser pulse depletes the Xe2Cl(4 2Γ) population by photoexcitation [as evidenced by the sudden suppression of the 4 2Γ → 1 2Γ blue-green (λ∼485 nm) fluorescence] and the absorption cross section is determined from the dependence of the degree of fluorescence suppression on the laser intensity. Contrary to the known Xe+21(1/2)u → 2(1/2)gUV absorption profile, Xe2Cl(4 2Γ) exhibits significant absorption at wavelengths as low as 193 nm, whereas the dimer ion absorption at 248 nm is only a fraction of its peak (λ∼340 nm) value. Photoionization of excited Xe2Cl at wavelengths below ∼275 nm may be responsible for the cross sections measured for photon energies as large as 6.4 eV. [ABSTRACT FROM AUTHOR]

The ultraviolet and visible absorption spectra of potassium vapor–xenon mixtures have been studied for [K] and [Xe] as large as 7×1017 and 4×1019 cm-3, respectively. Collision-induced absorption is observed in the ultraviolet (285≤λ≾370 nm) for the 4s → nf, nd (4≤n≤8) transitions but peak absorption lies an average of ∼0.6 nm to the red side of the position of the electric dipole forbidden line. The 4s → 4f, 5f transitions, in particular, are quite strong, having peak reduced absorption coefficients measured to be 1.1×10-39 and 2.6×10-40 cm5, respectively. Also, the KXe (C 2Σ) excimer has been observed in absorption for the first time. Absorption at the band peak (reduced absorption coefficient of ∼2×10-40 cm5 at λ=507 nm) is roughly two orders of magnitude weaker than that for the analogous bands of CsXe and RbXe and is displaced at least 5 nm to the blue side of the peak (∼520 nm) in the emission spectrum. Six reproducible undulations to the blue side of the absorption maximum display a periodicity of 111±17 cm-1. [ABSTRACT FROM AUTHOR]

The bound→free absorption band recently observed in a 114CdI laser discharge [Greene and Eden, Opt. Comm. (1985)] is shown to arise from absorption by the B 2Σ excited state of 114CdI to a previously unobserved level lying ∼4.3×104 cm-1 above ground. Pumping of the absorptive transition with a pulsed blue dye laser strongly suppresses gain (and, hence, laser output power) on the B→X band in the red (657 nm). Therefore, this constitutes the first known report of an ionic excited state→covalent absorption band [i.e., (M+X-)+hω→M*+X; M, X=metal and halogen atoms, respectively] for any of the group IIB metal halides or rare gas–halide laser molecules. Also, ωe for the B 2Σ state of 114CdI is >=110 cm-1 (considerably larger than an existing value in the literature) and the terminal level for this blue absorption band is tentatively assigned to a dissociative 2Σ state correlated with the Cd(5p 3PJ)+I separated atom limit. [ABSTRACT FROM AUTHOR]

The B 1Σ+u←a 3Πg transition of Cd2 (natural abundance) and 114Cd2 has been observed by laser excitation spectroscopy and analyzed. By exciting the Cd2 B←a transition in the visible (560≤λ≤730 nm) while monitoring B 1Σ+u→X 1Σ+g (bound→free) emission in the ultraviolet (∼270–310 nm), more than 40 red-degraded vibrational bands were recorded. Analysis of the spectrum has yielded vibrational constants for both the a 3Πg and B 1Σ+u states: ωe″=153.6±4.0 cm-1, ωe″xe″=0.52±0.06 cm-1, ωe′=105.3±1.0 cm-1, and ωe′xe′=0.44±0.03 cm-1. In addition, ΔRe≡ReB-Rea was determined to be 0.31±0.03 Å. Rotational structure has been partially resolved for 114Cd2 bands in the 620–655 nm and 719–723 nm regions, and the spontaneous emission lifetime of the a 3Πg state and the rate constant for quenching of Cd2 (a 3Πg) by collisions with background Cd atoms have been determined to be 8.6±2.5 μs and (2.2±0.3)×10-13 cm3 s-1, respectively. Also, analysis of the B→X emission (Condon internal diffraction) spectra produced when specific B 1Σ+u vibrational levels are populated has yielded ReX-ReB=0.95±0.02 Å. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Two color, pump–probe laser experiments on the ∼100 fs time scale have been applied to examining the dynamics of vibrational wave packets in the C 1Πu state of Cs2. Wave packets consisting of more than 20 C state vibrational levels are produced with an initial pulse having a central wavelength between ∼620 and 680 nm. The temporal history of the wave packet is inferred from the time and energy-integrated photoelectron signal produced when the excited state is photoionized by a time-delayed pulse centered at 605, 610, 615, 617, or 620 nm. Because of the difference in equilibrium internuclear separations for the Cs2(C) and Cs+2(X) states (ΔRe≊0.75 Å), wave packets are readily observed (signal-to-noise ratio >=10) without the need to resort to mass selection techniques. Frequency analysis of the wave packet data shows a dominant (fundamental) component that decreases from 29 to ∼28.3 cm-1 as the pump wavelength is tuned from 627 to 641 nm. Other spectral modes at ∼23.5 and ∼34 cm-1 and a weaker term at twice the fundamental frequency are also observed and quantum mechanical calculations of the wavepacket motion are in accord with the experimental results. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The photoabsorption spectrum of the Kr+2 ground electronic state (X 2Σ+u) has been measured in the ultraviolet (257–355 nm) by a fluorescence suppression technique. Peak absorption is recorded at ∼330 nm and the spectral width of the observed continuum is ∼60 nm (FWHM) which is ∼25% smaller than the value predicted by theory for a Kr+2(X) vibrational distribution thermalized at 300 K. Similar experiments with Ar/Kr mixtures: (1) reveal weak absorption attributed to ArKr+ peaking at ∼280 nm, and (2) suggest that the ArKr+ ion is rapidly converted to Kr+2 by a displacement reaction. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Rotationally resolved transitions from the metastable 4sσ a3Σ+u state of Ar2 to the 7pσ 3Σ+g state have been observed by laser excitation spectroscopy in a pulsed corona discharge. Laser induced fluorescence spectra for the 7pσ 3Σ+g←a3Σ+u transition observed near 19 530 cm-1 have led to the first resolved triplet splittings and rotational analyses for Rydberg–Rydberg transitions in Ar2. Analyses for the (0–0), (0–1), (1–0), and (1–1) bands of 7pσ←a are presented here. Bands with v’=2 are observed for this system but exhibit no rotational structure and bands with v’>=2 are not observed, indicating that the predissociation lifetimes of these upper levels fall rapidly for v’>=1. Data presented lead to rotational energies and molecular constants for the a3Σ+u and 7pσ 3Σ+g states, including the effective rotational constant Be which is determined to be 0.1412 cm-1 and 0.1345 cm-1 for the 4sσ a3Σ+u and 7pσ 3Σ+g states, respectively. Results are compared to ab initio and other calculations, and combined with existing data to produce experimental estimates of D0 and De for the a3Σ+u state of 5700±200 cm-1 and 5850±200 cm-1, respectively. [ABSTRACT FROM AUTHOR]

Multiphoton ionization and fragmentation of the hydrogen iodide molecule have been examined at 355 and 532 nm and optical field intensities of 1011-5×1012 W cm-2 by photoelectron spectroscopy and time-of-flight mass spectrometry. At 532 nm, scission of the H–I bond is favored over direct photoionization of the molecule. Three and four photon ionization of HI at 355 nm (hω≊3.5 eV), however, reveals clear vibrational progressions associated with the HI+ 2Π3/2 and 2Π1/2 ion product states. Analysis of these spectra yields vibrational frequencies for the 2Π3/2 and 2Π1/2 states that agree to better than 8% with those determined by Böwering et al. [Chem. Phys. Lett. 189, 467 (1992)] from single photon ionization experiments. Above threshold ionization (ATI) of both HI and atomic iodine are also observed. [ABSTRACT FROM AUTHOR]

The bound←free absorption (photoassociation) of thermalized Kr(4p6 1S0)–F(2p5 2P) collision pairs has been studied in the ultraviolet in Kr/F2 gas mixtures by laser pump–probe techniques. Experiments carried out at 248 nm with two time-delayed excimer laser pulses demonstrate that KrF(B2 Σ1/2+) molecules are produced by the photodissociation of F2, followed by the photoassociation of Kr–F collision pairs by a single ∼5 eV photon. The saturation behavior of this photoassociation process indicates that the currently accepted value for the KrF(B→X) stimulated emission cross section (2.6×10-16 cm2) is too large by at least a factor of 2. A revised value for σSE of (1.2±0.6)×10-16 cm2 is suggested. By indirectly monitoring the temporal decay of the atomic fluorine number density owing to reformation of F2, the rate constant for the recombination of F atoms in a Kr background by three-body (2F+Kr) collisions has been determined to be kr=2×10-32 cm6 s-1. [ABSTRACT FROM AUTHOR]

Spectra attributed to transitions from the 5sσ a 3Σu+ metastable state of Kr2 to excited Rydberg states have been observed by intracavity and multipass absorption and by laser excitation spectroscopy. Four distinct Rydberg series converging on the X 2Σu+ core of Kr2+ have been identified. Rotational structure is not resolved in any of the observed spectra. Most of the spectra can be assigned to vibronic transitions in the nfλ(3Πg,3Σg+) and npλ(3Πg,3Σg+)←a 3Σu+ series by analogy with similar spectra in the lighter rare gas dimers. The 6pπ 3Πg←a 3Σu+ and 8pσ 3Σg+←a 3Σu+ transitions observed in the ∼19 200–18 200 cm-1 region exhibit many bands with v’≠0 in their (v’–v‘) vibrational band system developments. The higher n value members of these and other observed series are dominated by (0–0) transitions. All series are perturbed in a few energy regions by states built on the excited ion core states A 2Π3/2g and B 2Π3/2u. Laser excitation spectra for the 8pσ 3Σg+←a transition obtained by monitoring various 5p→5s emission lines of atomic krypton suggest that the 8pσ 3Σg+ Rydberg state of Kr2 is predissociated by a repulsive potential correlated with Kr 5p[3/2]1+Kr(1S0). The lowest ionization limit of Kr2 (relative to a 3Σu+,v=0) was determined to be 28 484±5 cm-1. [ABSTRACT FROM AUTHOR]

Photoassociation, the bound←free absorption of a photon by a colliding pair of atoms, has been studied experimentally in the Kr–F and Xe–I systems and is demonstrated to be an effective tool for deducing molecular interaction potentials and for determining the dependence of the relative electronic transition moment on internuclear separation (R) in small molecules having dissociative ground states. Specifically, the excitation (‘‘action’’) spectrum for the photoassociation of thermalized Kr(4p6 1S0)–F(2p5 2P) collision pairs, measured at 300 K in the 208–250 nm wavelength region, reveals deeply modulated Franck–Condon structure associated with transitions of Kr–F pairs from the ground state’s thermal and vibrational continua to the lowest 27 bound vibrational states (v’=0–26) of the KrF(B 2Σ) excited state. Under these conditions, photoassociation occurs in a narrow (ΔR=0.7–0.8 Å) Franck–Condon region in which the difference potential is a single valued function of R. Similar spectra have been observed for Xe(1S0)–I(2P) pairs in the 208–258 nm spectral region that arise from bound←free transitions to the lowest ∼40 vibrational levels of the XeI(B) state. Since the Kr–F and Xe–I collision pairs are photoexcited from the vibrational continuum of the ground state, the atomic pair energy distribution is thermalized and the envelope of the excitation spectrum can be interpreted directly in terms of the relative B–X transition moment, μ(R). Consequently, the behavior of μ(R) has been determined over a limited range in R and, for XeI, μ is found to fall by a factor of 2 in the ∼0.4 Å region extending from R-ReB≊0.25–0.62 Å. The XeI ground state (X 2Σ1/2+) potential in the 3.0≤R≤5.0 Å interval, derived from photoassociation... [ABSTRACT FROM AUTHOR]

Extensive spectra attributable to transitions from the 4sσ a 3Σ+ metastable state of Ar2 to excited Rydberg states have been observed by intracavity absorption spectroscopy and by laser excitation spectroscopy in the afterglow of a pulsed corona discharge. Of these the most extensive and best resolved were the laser induced fluorescence spectra. Most of the spectra can be assigned to vibronic transitions in the nfλ (3Πg,3Σ+g) and npλ (3Πg,3Σ+g)←a 3Σ+u series. The 5pπ3Πg←a 3Σ+u and 7pσ 3Σ+g←a 3Σ+u transitions observed near 19 823 and 19 529 cm-1, respectively, exhibit many bands with v’≠0 in their (v’–v‘) vibrational band system developments. The higher n value members of these and other observed series are dominated by (0–0) transitions. Rotational structure is partially resolved in a few bands of the 7pσ 3Σ+g–a system, but most of the spectra observed appear to be either rotationally unresolved or made up of blended collections of rotational lines. Above the 7pσ, 5pπ pair, the (n+2)pσ 3Σ+g and npπ 3Πg members of the npλ series rapidly coalesce, indicating a rapid onset of decoupling of the electronic orbital angular momentum, L, from the internuclear axis.Such decoupling leads to np-complex formation at n values much lower than observed in the npλ series of the lighter dimers: He2 and Ne2. From the data for the observed series, the lowest ionization limit of Ar2 (relative to a 3Σ+u,v=0 ) was determined to be 29 373±3 cm-1. Vibrational intervals ΔG(v+1/2) for a 3Σ+u(v≤4) and 7pσ 3Σ+g(v≤2) yield the vibrational constants ωe=296 and 282 cm-1 and xωe=2.5 and 17 cm-1, respectively. [ABSTRACT FROM AUTHOR]

Presents a study that described a series of experiments in which the stability and emission spectra of several dye vapors in an ultraviolet-preionized, transverse discharge. Experimental apparatus; Results and discussion; Summary.

Focuses on a study which examined rare gas-halide and rare gas-oxide excimer emission using a coupling of a pulsed electric discharge to a supersonic free jet expansion. Effect of a modification in the placement of the discharge anode; Discussion on argon and xenon fluoride fluorescence emission waveforms; Results of laser-induced fluorescence probing of argon metastable species.

Focuses on a study which investigated the growth of gallium arsenide irradiated with ultraviolet laser light in a metalorganic chemical vapor deposition reactor. Information on laser photochemical vapor deposition; Methodology of the study; Results and discussion.

Presents a study which examined germanium films grown on gallium arsenide by laser photochemical vapor deposition in parallel geometry. Experimental details; Results and discussion; Conclusion.

Presents a study which examined the activation energy and spectroscopy of the growth of germanium (Ge) films by ultraviolet laser-assisted chemical vapor deposition. Analysis of the spatially and temporally resolved emission profiles for the GeH and atomic Ge photofragments; Description of experimental apparatus and data acquisition procedures; Measurement of the activation energy for the film growth process.

Cs atomic lasers, operating on the D2 line and pumped by the photoassociation of Cs-rare gas thermal collision pairs, have been examined in mixtures of Cs vapor with Ar, Kr, or Xe. Photoexcitation of the B 2Σ+ − X 2Σ+ molecular transition (i.e., the blue satellite of the D2 transition) at 836.7 nm (Cs-Ar), 841.1 nm (Cs-Kr), or 842.7 nm (Cs-Xe) yields lasers at 852.1 nm whose characteristics (optical-to-optical conversion efficiencies, pump energy threshold, and temperature dependence) are a reflection of the structure of the B 2Σ+ interatomic potential associated with each Cs-rare gas pair. Output pulse energies above 100 μJ are obtained from the Cs-Ar complex in the 493–513 K interval because of the height of the B 2Σ+ barrier in the Franck-Condon region for the pump, a molecular parameter also responsible for the robust temperature stability of the laser. Slope efficiencies (with respect to absorbed pump energy) of 17%, 12%, and 27% have been measured for Cs-Ar, Cs-Kr, and Cs-Xe pairs at 473 K, 453 K, and 453 K, respectively. The data reported here firmly link the performance of a photodissociation laser with the structure of the intermediate diatomic complex. [ABSTRACT FROM AUTHOR]

Small arrays (3×3→10×10) of microcavity plasma devices, having a two-stage structure comprising multilayer stacks of nanostructured Al2O3 grown on Al foil with cylindrical microcavities 100–300 μm in diameter, produce intense ultraviolet (UV) and visible emission when operated in Ar/3% N2 gas mixtures or Ne and excited by a sinusoidal ac voltage wave form. Near-UV (300–400 nm) intensities emitted into a solid angle of ∼10-2 sr above 30 mW per cm2 of radiating area are measured for Ar/N2 gas mixture pressures of 400–800 Torr, an excitation frequency of 15 kHz, and an average current >=20 mA (rms). Two or more conjoined cylindrical microcavities of differing diameters allow for the confinement of the plasma to progressively smaller volumes as the power deposited is increased (thereby decreasing the Debye length and the spatial extent of the cathode fall region) and provides a convenient tool with which the luminosity of an array can be modulated. The transition of a microplasma from confinement in a 200-μm-diam cavity into an adjacent 100-μm-diam microcavity is accompanied by an order-of-magnitude increase in the luminance of Ne plasmas in a 3×3 array as the rms current is increased by ∼1 mA. [ABSTRACT FROM AUTHOR]

More than 25 W of average power and >800 W of peak power have been generated at λ = 172 nm (hν = 7.2 eV) in the vacuum ultraviolet (VUV) from the Xe2 molecule in flat, 10 × 10 cm² lamps having an active area and volume of 80 cm2 and <60 cm³, respectively. Powered by at least two interlaced arrays of microplasmas generated within cavities fabricated into an interior surface of the <6 mm thick lamp, these lamps have an electrical-to-optical conversion efficiency >20%. For a bipolar voltage waveform driving frequency of 137 kHz and a 54% Xe/Ne gas fill mixture at a 300 K pressure of 550 Torr, the lamp generates as much as 31.5 W of average power and intensities >350 mW cm-2 in 40-60 µJ, 70±10 ns FWHM pulses produced in a burst mode-four pulses of 600-850Wpeak power in every cycle of the driving waveform. The lamp intensity is uniform to within ±2.5% at ≥10mm from its surface and average power varies linearly with pulse repetition frequency throughout the 18-135 kHz interval. The spectral breadth of the Xe dimer emission is ~9nm FWHM and time-resolved, spatial intensity maps show improved utilization of the power pulse (V⋅I) with two or more microcavity arrays that are interleaved. This photonic source technology is capable of generating unprecedented power levels in the VUV spectral region (e.g., ~2.5 kW m-2) with tiled lamps. [ABSTRACT FROM AUTHOR]

The ultraviolet emission spectrum of GeH has been examined following the two photon-initiated dissociation of germane (GeH4) at 248 nm. The most prominent peaks lying between ∼250 and 340 nm are tentatively assigned to B 2Σ(v′=0)→X 2Π1/2,3/2 (v′) transitions. From this premise, the vibrational constants for the X 2Π states have been estimated to be ωe=(1949±5) cm-1, ωexe=16.5±2.0 cm-1, and ωeye=(0.7±0.3) cm-1. Also, the radiative lifetime of the lowest vibrational levels of the GeH A 2Δ state in transitions to ground has been measured to be 12±2 ns. [ABSTRACT FROM AUTHOR]

Direct coupling of the optical field in a ~244 nm thick, CdSe/ZnS quantum dot film to an optical fiber has yielded lasing in the red (λ ~ 644 nm) with a threshold pump energy density < 2.6 mJ cm-2. Comprising 28-31 layers of ~8 nm diameter quantum dots deposited onto the exterior surface of a 125 µm diameter coreless silica fiber, this free-running oscillator produces 134 nJ in 3.6 ns FWHM pulses which correspond to 37 W of peak power from an estimated gain volume of ~4.5 × 10-7 cm³. Lasing was confirmed by narrowing of the output optical radiation in both the spectral and temporal domains, and the laser beam intensity profile approximates a top hat. [ABSTRACT FROM AUTHOR]

Two platforms developed to probe the plasma–solid interface are briefly reviewed here. A sensitive diagnostic of the interaction between a low-temperature plasma and a silicon surface is provided by the plasma bipolar junction transistor (PBJT) in which e−–h+ (semiconductor) and e−–ion (gas phase) plasmas are separated by a nanoscale potential barrier but coupled by a strong (>1 V µm−1) electric field. Electrical properties of the Si base–collector plasma interface are controlled by the bias applied to the PBJT emitter-base junction, as well as the morphology and electronic structure of the base surface facing the collector plasma. Recent experiments are described in which the Si(100) surface of the PBJT base is transformed into black Si, a nanostructured array comprising ∼3 × 109 nanocones. Each cone is ∼100 nm in height and has a radius of curvature at its tip of <1 nm. Altering the base surface so as to intentionally enhance field emission results in burst-mode operation of the PBJT in which the collector current oscillates as a result of the periodic contraction (or collapse) and revival of the plasma sheath. The observed oscillation frequencies (3.6–13 kHz) are consistent with the transit time of ions across the sheath. Integration of a microplasma device with a transmission electron microscope has also been realized recently (Tai et al 2013 Sci. Rep.3 1325). This diagnostic tool allows plasma–surface interactions to be observed in real-time with a spatial resolution <100 nm. Although initial experiments concerned the growth of Au islands in a dc plasma, the extension of this probe to nanostructured and spatially modulated surfaces is proposed. [ABSTRACT FROM AUTHOR]