Young S. Choi

Formation of efficient dye-sensitized solar cells by introducing an interfacial layer of long-range ordered mesoporous TiO2 thin film.

Long-range ordered cubic mesoporous TiO 2 films with 300 nm thickness were fabricated on fluorine-doped tin oxide (FTO) substrate by evaporation-induced self-assembly (EISA) process using F127 as a structure-directing agent. The prepared mesoporous TiO 2 film (Meso-TiO 2) was applied as an interfacial layer between the nanocrystalline TiO 2 film (NC-TiO 2) and the FTO electrode in the dye-sensitized solar cell (DSSC). The introduction of Meso-TiO 2 increased J sc from 12.3 to 14.5 mA/cm (2), and V oc by 55 mV, whereas there was no appreciable change in the fill factor (FF). As a result, the photovoltaic conversion efficiency ( eta) was improved by 30.0% from 5.77% to 7.48%. Notably, introduction of Meso-TiO 2 increased the transmittance of visible light through the FTO glass by 23% as a result of its excellent antireflective role. Thus the increased transmittance was a key factor in enhancing the photovoltaic conversion efficiency. In addition, the presence of interfacial Meso-TiO 2 provided excellent adhesion between the FTO and main TiO 2 layer, and suppressed the back-transport reaction by blocking direct contact between the electrolyte and FTO electrode.

Quasistable extreme motion vibrational states of HFCO above its dissociation threshold

The anharmonic mixing of highly excited vibrational states of HFCO above its dissociation threshold is studied by stimulated emission pumping spectroscopy. At 0.05‐cm−1 resolution, individual molecular eigenstates were resolved and state mixings observed by the distribution of oscillator strength to nearby dark states of the background. Most of the zero‐order vibrational levels observed in the energy range between 13 000 and 23 000 cm−1 are assigned to long Franck–Condon‐allowed progressions of extremely high overtones of the C–H out‐of‐plane bending mode (ν6) in combination with the C■O stretching mode (ν2). The extent of state mixing of highly excited vibrational states is strongly mode dependent. For vibrational states with almost the same total vibrational energy, states with the most quanta in ν6 show the least coupling with other states. More strikingly, as the total energy is increased by adding ν6 quanta, the state mixing becomes weaker. For the vibrational states observed above 18 000 cm−1, those...

State‐specific unimolecular reaction dynamics of HFCO. I. Dissociation rates

Rovibrationally resolved unimolecular reaction rates of highly vibrationally excited HFCO (S0) are measured and found to be strongly state specific in the energy range of 14 500 cm−1–23 000 cm−1. HFCO molecules are excited to single rovibrational levels in the tunneling region by stimulated emission pumping (SEP) and the dissociation rate of each level is measured by monitoring the temporal evolution of its population by laser‐induced fluorescence. The dissociation rates increase by a factor of 10 to 100 or more for small increases in rotational quantum number from J=0 up to J=4 and Ka=2. The dependence on Ka is the strongest. At higher energies, where dissociation lifetimes are shorter than the laser pulse duration, dissociation rates are estimated from the linewidths of well‐resolved transition lines measured by high‐resolution SEP spectroscopy. In this energy region, dissociation rates are also dependent upon rotational state but much less strongly than in the tunneling region. Vibrational mode specificity in the dissociation rates is observed. For states with approximately the same total energy those with higher excitation in the out‐of‐plane bending mode (ν6) dissociate more slowly than others. For rotationless levels in the tunneling region, the A’ states decay much more rapidly than the A‘ states indicating symmetry‐induced mode specificity.The dissociation rates in HFCO almost surely exhibit mode specificity because the ν6 mode is weakly coupled to the reaction coordinate. The enhancement of this coupling by Coriolis forces appears to produce the unprecedentedly large rotational level dependence of the rates. However, the spectroscopically observed coupling of ν6 to the background of vibrationally mixed levels is not significantly increased by rotation. Thus states with nearly all of the excitation energy in the reaction coordinate appear not to be strongly mixed into the background states. The large effect of rotation on the rates thus seems to result from Coriolis coupling of extreme motion in the reaction coordinate to the background levels or directly to ν6. By applying the Rice–Ramsperger–Kassel–Marcus theory to the measured dissociation rates for high rotational states, the barrier height for the molecular dissociation of HFCO to HF+CO is estimated to be 49±4 kcal/mole.Rovibrationally resolved unimolecular reaction rates of highly vibrationally excited HFCO (S0) are measured and found to be strongly state specific in the energy range of 14 500 cm−1–23 000 cm−1. HFCO molecules are excited to single rovibrational levels in the tunneling region by stimulated emission pumping (SEP) and the dissociation rate of each level is measured by monitoring the temporal evolution of its population by laser‐induced fluorescence. The dissociation rates increase by a factor of 10 to 100 or more for small increases in rotational quantum number from J=0 up to J=4 and Ka=2. The dependence on Ka is the strongest. At higher energies, where dissociation lifetimes are shorter than the laser pulse duration, dissociation rates are estimated from the linewidths of well‐resolved transition lines measured by high‐resolution SEP spectroscopy. In this energy region, dissociation rates are also dependent upon rotational state but much less strongly than in the tunneling region. Vibrational mode specifi...

The 2 1Ag state of trans,trans‐1,3,5,7‐octatetraene in free jet expansions

One‐ and two‐photon fluorescence excitation and emission spectra of the S1↔S0 transition of trans,trans‐1,3,5,7‐octatetraene have been measured for the first time in free jet expansions. The one‐photon excitation spectrum is the same, with the exception of significant differences in the intensities of a few lines, as the two‐color, resonance‐enhanced, two‐photon ionization spectrum, previously assigned to the 2 1A’←1 1A’ transition of cis,trans‐1,3,5,7‐octatetraene. However, comparison of the one‐ and two‐photon fluorescence excitation spectra shows clearly that the carrier of the spectrum has inversion symmetry, as expected for trans,trans‐1,3,5,7‐octatetraene. The one‐photon spectrum is built on bu Herzberg–Teller promoting modes, which are origins of progressions in ag modes, while the two‐photon spectrum is due to a single progression in ag modes starting from the 2 1Ag←1 1Ag electronic origin. The appearance of out‐of‐plane vibrations, possibly including torsions of the polyene framework, suggests la...

Determination of the singlet/triplet branching ratio in the photodissociation of ketene

The rotational distributions of CO products from the dissociation of ketene at photolysis energies 10 cm−1 below, 56, 110, 200, 325, 425, 1107, 1435, 1720, and 2500 cm−1 above the singlet threshold (30 116.2 cm−1 ), are measured in a supersonic free jet of ketene. The CO(v‘=0) rotational distributions at 56, 110, 200, 325, and 425 cm−1 are bimodal. The peaks at low J’s, which are due to CO from the singlet channel, show that the product rotational distribution of CO product from ketene dissociation on the singlet surface is well described by phase space theory (PST). For CO(v‘=0) rotational distributions at higher excess energies (1107, 1435, 1720, and 2500 cm−1 ), the singlet and triplet contributions are not clearly resolved, and the singlet/triplet branching ratios are estimated by assuming that PST accurately predicts the CO rotational distribution from the singlet channel and that the distribution from the triplet channel changes little from that at 10 cm−1 below the singlet threshold. The singlet yield shows a rapid increase in the low excess energy region (0–300 cm−1 ), and a slower increase above. The singlet and triplet rate constants are derived from the directly measured total rate constants using the singlet yields. The triplet rate constant increases monotonically with increasing photolysis energy through the singlet threshold region. The singlet rate constant is accurately established in the threshold region and found to increase much less rapidly than predicted by phase space theory. At 2500 cm−1 excess energy, the CO(v‘=1) rotational distribution is obtained, and the ratio of CO(v‘=1) to CO(v‘=0) products for the singlet channel is measured to be 0.045±0.017. This ratio is close to the variational Rice–Ramsberger–Kassel–Marcus (RRKM) calculation 0.038, and the separate statistical ensembles (SSE) prediction 0.041, but much greater than the PST prediction, 0.016.

Evidence for mode specific dynamics in the stimulated emission pumping spectra of HFCO

Stimulated emission pumping spectra of HFCO in a pulsed jet at 0.2 cm−1 resolution in the range of 13 000 to 23 000 cm−1 are reported. Above the dissociation threshold, ≊14 000 cm−1, linewidths vary from 0.15 to 2.0 cm−1 for S0 vibrational levels within an energy range of a few hundreds of cm−1 . Substantial vibrational mode dependence of intramolecular vibrational energy redistribution and/or dissociation rates is likely.

State‐specific unimolecular dissociation dynamics of HFCO. II. CO rotational distribution and Doppler widths

Rovibrational state distributions and Doppler widths of CO fragments formed from unimolecular dissociation of HFCO in its ground electronic state are measured by vacuum ultraviolet laser‐induced fluorescence, following state‐selective preparation of the molecule in a single quantum state in the energy region of 2000 to 3000 cm−1 above the dissociation threshold by stimulated emission pumping. CO fragments are rotationally hot and distributed over J≤15 to J=63 with distributions peaking at J=45 to 50 depending upon the initial HFCO dissociative state. Although CO rotational distributions are significantly different for different initial states, about 20% of the total available energy is released on average as rotational energy of CO for all three initial states studied. The yield of CO(v=1) fragments is determined to be about 10% and CO(v≥2) fragments are not observed. The average Doppler width of CO fragments is 0.85 cm−1, which indicates that ∼50% of the total available energy is released as translation....

One‐ and two‐photon fluorescence excitation spectra of the 2 1Ag states of linear tetraenes in free jet expansions

One‐ and two‐photon fluorescence excitation spectra of the S1←S0 transitions of the all‐trans isomers of 1,3,5,7‐octatetraene, 1,3,5,7‐nonatetraene, and 2,4,6,8‐decatetraene have been obtained in free jet expansions. Comparison of the one‐ and two‐photon spectra allows the unambiguous identification of electronic and vibronic origins and, for octatetraene and decatetraene, provides clear evidence for molecular inversion symmetry. One‐photon spectra show ag progressions built on Herzberg–Teller, bu promoting modes, while two‐photon spectra are built on progressions of ag modes starting from the 2 1Ag←1 1Ag electronic origins. In nonatetraene, the absence of inversion symmetry results in an allowed electronic origin in both the one‐ and two‐photon spectra. Nevertheless, bands built on vibronic origins dominate the one‐photon spectrum. The S1←S0 spectra of nonatetraene and decatetraene exhibit characteristic splittings of vibronic bands that can be quantitatively explained by the tunneling of the methyl grou...

The Relationship Between Pre-Retirement Earnings and Health Status in Old Age

Abstract It is widely believed that people who earned more while they were young tend to be healthier in old age. Our study, the findings of which are presented in this article, questioned whether this generalization applies equally to black and white elderly people. A multivariate analysis, which included gender, marital status, age, education, and number of children raised as control variables, indicated that the level of earnings during peoples working lives is not related to the health impairments of black people at the time of retirement or 10 years later, but strongly related to the health impairments of white people.