Stephen H. Ashworth

Light-emitting diode (LED) polymerisation of dental composites : Flexural properties and polymerisation potential

The clinical performance of light polymerised dental composites is greatly influenced by the quality of the light-curing unit (LCU) used. Commonly used halogen LCUs have some specific drawbacks such as decreasing of the light output with time. This may result in low degree of monomer conversion of the composites with negative clinical implications. Previous studies have shown that blue-light-emitting diode (LED) LCUs have the potential to polymerise dental composites without having the drawbacks of halogen LCUs. Despite the relatively low irradiance of current LED LCUs, their efficiency is close to that of conventional halogen LCUs with more than twice the irradiance. This phenomenon has not been explained fully yet. Hence, more tests of the LED LCUs effectiveness and of the mechanical properties of oral biomaterials processed with LED LCUs need to be carried out. This study investigates the flexural properties of three different composites with three different shades, which were polymerised with either a commercial halogen LCU or an LED LCU, respectively. In most cases no significant differences in flexural strength and modulus between composites polymerised with a halogen LCU or an LED LCU, respectively, were found. A simple model for the curing effectiveness based on the convolution absorption spectrum of the camphorquinone photoinitiator present in composites and the emission spectra of the LCUs is presented.

Vibrational analysis of the 460 nm band system of nickel dichloride produced in a free jet expansion

Using a free‐jet expansion which incorporates a heated nozzle, we have recorded the laser excitation spectrum of the 460 nm band system of NiCl2 at a rotational temperature of ∼40 K. 35Cl/37Cl isotope shifts were resolved which permit the assignment of progressions involving the symmetric stretching vibrational mode and identify a triplet splitting with spacings of 96 and 149 cm−1 which is believed to be due to spin–orbit coupling. Sequence bands involving the bending vibrational mode are also tentatively assigned. Only a small change in the symmetric stretching vibrational wave number is found between the electronic states involved in this transition (ν’1 =356 cm−1, ν″1 =360 cm−1). This result and the triplet splitting observed are discussed with respect to the possible electronic states involved and the assignment of this band system as either a Laporte forbidden g↔g transition involving the d orbitals on the Ni atom or an allowed u↔g charge transfer transition.

Rotational analysis of bands in the 460 nm system of nickel dichloride produced in a free-jet expansion: Determination of the structure and electronic ground state of nickel dichloride

By use of a free‐jet expansion which incorporates a heated nozzle, we have recorded the laser excitation spectrum of the 460 nm band system of NiCl2 at rotational resolution. The rotational temperature in these recordings was about 12 K. Several bands have been recorded and analyzed for three isotopomers, 58Ni35Cl2, 60Ni35Cl2, and 58Ni35Cl37Cl in natural abundance. Spin components with Ω values of 0 and 1 have been identified in both the upper and lower states of the transition. Accurate values for all three vibrational intervals ν1, ν2, and ν3 have been determined for nickel dichloride in the upper state and for the bending wave number ν2 in the lower state. The results show that the molecule is linear in both states involved in the transition and that the lower (ground) state is 3Σ−g in character. Evidence is presented from the nickel isotope shifts to show that the transition is vibronically induced through the bending vibration and that the upper state is vibronically 3Πu in character; it probably der...

Vibronic excitations of large molecules in solution studied by two‐color pump–probe experiments on the 20 fs time scale

The ultrafast vibronic response of organic dye molecules in solution is studied in pump–probe experiments with 30 fs excitation pulses resonant to S0–Sn transitions. The molecular dynamics is probed either by pulses at the same spectral position or by 20 fs pulses overlapping with both the S0–S1 absorption and emission bands. Three contributions on distinctly different time scales are observed in the temporally and spectrally resolved two‐color measurements. In the regime below 50 fs, a strong coherent coupling of the S0–Sn and the S0–S1 transitions occurs that is due to coherent vibrational motions in the electronic ground state. This signal is superimposed on the fast bleaching of the electronic ground state, resulting in a steplike increase of transmission. In the range of the S0–S1 emission band, one finds a subsequent picosecond rise of transmission that is due to stimulated emission from vibronic S1 states. The data demonstrate that the relaxation of Sn states directly populated by the pump pulses i...

Generation of 16-fs pulses at 425 nm by extracavity frequency doubling of a mode-locked Ti:sapphire laser

Summary form only given. We report the generation of 16 fs pulses at 425 nm by second-harmonic generation using 15 fs pulses at 850 nm and a 100 /spl mu/m BBO crystal. The average fundamental power at the crystal was 550 mW and the second-harmonic power generated was 40 mW. Having expanded the fundamental beam from a self-mode-locked Ti:sapphire laser in a 31 mirror telescope it was then focused into a BBO crystal with a f=75 mm lens. The second harmonic was recollimated, compressed in a double-pass quartz prism compressor and the autocorrelation measured. Maximum output power was achieved when the fundamental at the doubling crystal had a minimum pulse length.

Detection of the N3 free radical by laser magnetic resonance at 6.08 μm

Abstract Lines in the ν 3 (antisymmetric stretch) fundamental band of the N 3 radical in the X 2 Π g state have been detected by CO laser magnetic resonance. The observations have been assigned to P, Q, and R lines in the fundamental vibration-rotation band and lead to a precise determination of the vibrational interval, the ground-state rotational constant and its dependence on the antisymmetric stretching vibration: ν 3 = 1644.67903(24) cm −1 , B 0 =0.431438(16) cm −1 and α 3 =0.004357(18) cm −1 .

The far-infrared laser magnetic resonance spectrum of the SH radical

Abstract We have extended the observations in the far-infrared laser magnetic resonance spectrum of the SH radical in its ground 2 Π state to cover the first three rotational transitions in the 2 Π 3 2 component. Signals were seen from all naturally occurring isotopes of sulfur except the least abundant, 36 S (0.02%). The measurements have been analyzed, together with selected data from previous work, to determine an improved set of molecular parameters for 32 SH in its ground state. The implications of these parameters are discussed. The magnetic and electric nuclear hyperfine parameters for 33 SH are significantly improved over previous work. The frequencies of rotational transitions in 32 SH in the absence of a magnetic field are calculated from the parameter set to aid in the detection of this radical in astrophysical sources.

An injection seeded narrow bandwidth pulsed optical parametric oscillator and its application to the investigation of hyperfine structure in the PF radical

We describe the construction of an all solid-state, narrow bandwidth, pulsed optical parametric oscillator (OPO) based on β-barium borate nonlinear crystals. The OPO was injection seeded by an external cavity diode laser in the range 755–855 nm to generate high power narrow bandwidth tunable light in this range and simultaneously at 606–669 nm. The bandwidth of the visible light was ∼130 MHz, and after frequency doubling or sum frequency mixing with the second harmonic of the pump Nd:YAG laser, sub-Doppler spectra with an overall resolution of 450 MHz were taken in the UV. The system is demonstrated by taking high-resolution spectra of the v′=2–3 and 5–7 bands of the A 3Π–X 3Σ−(v′,0) progression and the v′=4–v″=0 band of the d 1Π–a 1Δ transition in PF. These spectra show clear hyperfine structure, and an analysis of this structure is presented and interpreted in terms of the electronic structure of the molecule. As a prelude to this high-resolution study, the first ten members of the A–X band system and t...

Vibrational and vibronic dynamics of large molecules in solution studied on a 20 fs timescale

Abstract The ultrafast dynamics of organic dye molecules in solution are studied in temporally and spectrally resolved experiments with 30 fs pump pulses resonant to short-wavelength S 0 −S n transitions and 20 fs probe pulses overlapping with both the S 0 −S 1 absorption and emission bands. Around zero delay, a pronounced coherent signal occurs with sign and amplitude depending on the spectral position within the probe pulses. The coherent coupling of the type electronic transitions is due to the coherent vibrational motion induced by an impulsive Raman-type excitation in the electronic ground state. The molecules promoted to S n states give rise to a fast ground state bleaching on a 50 fs timescale that is followed by the picosecond onset of stimulated emission after accumulation of the excited molecules at the bottom of the S 1 state.

The high resolution Fourier-transform chemiluminescence spectrum of the HS2 radical

The chemiluminescence spectrum of the HS2 radical has been recorded with a Fourier-transform spectrometer. The overview spectrum in the region 9000 cm−1 to 4000 cm−1 has been analysed and the vibrational parameters obtained are presented. In addition three of the bands ( , and ), which have been recorded at high resolution, have been rotationally analysed and the results of the fit are presented. The results are discussed in the context of previous theoretical and experimental studies.