Increasing Excitation Energy Research Articles

Role of Singles Amplitudes in ADC(2) and CC2 for Low-Lying Electronically Excited States.

The closely related second-order methods CC2 and ADC(2) usually perform very similarly for single excitations of organic molecules. However, as rationalized in this work, significant deviations between these two methods can arise if the ground state and a low-lying singly excited state arise from a strong coupling between their leading configurations. Such a configuration mixing is partially accounted for in CC2 through the ground-state singles amplitudes but is omitted in ADC(2). This can cause unusual deviations between the results obtained with these methods. In this work, we study how severe this effect can become at the example of two solvatochromic dyes: the negatively solvatochromic betaine dye N1-tBu and the positively solvatochromic bithiophene P1. These two dyes allow one to study the limits of both small and somewhat larger excitation energies and configuration mixing by tuning the S0 → S1 transition energy through the polarity of the environment. Higher-level calculations at the CC3 level provide information on the accuracy of ADC(2) and CC2 in these cases. The most extreme deviation between ADC(2) and CC2 is found for N1-tBu in vacuum, where the ADC(2) result is 0.45 eV below that of CC2. In this case, the methodical error of CC2 with respect to CC3 is only 0.05 eV. With increasing excitation energy in polar solvents, the CC2-ADC(2) deviation decreases and reaches a value of only 0.15 eV. For P1, which has larger excitation energies, these effects are reversed due to the opposite solvatochromism but also smaller in magnitude: the deviation increases from 0.08 eV in vacuum to 0.16 eV in the so-called conductor limit of the continuum solvation model. Although for these two dyes larger deviations are observed for smaller excitation energies, the extent of configuration mixing does not generally correlate with only the size of excitation energy. For example, s-triazine (0.15 eV), formamide (0.19 eV), and formaldehyde (0.23 eV) also show large deviations between CC2 and ADC(2) despite their much higher excitation energies compared to those of N1-tBu and P1.

Multiexciton spectra of molecular aggregates: application to photosynthetic antenna complexes.

We perform theoretical studies of nonlinear spectral responses of molecular aggregates upon multiple electronic excitations. It is shown that the transient absorption (TA) spectra exhibit gradual shifting to short wavelengths upon an increase in excitation energy accompanied by population of higher-order exciton manifolds. This transformation of the TA profile reflects a character of the exciton splitting and, therefore, is strongly dependent on the aggregate shape and size as well as on the exciton couplings and disorder of the site energies. The theory is applied for modeling of the intensity-dependent TA spectra of a light-harvesting LH1 antenna from a photosynthetic purple bacterium. Fitting of the data allowed verification of the exciton model of the complex (enabling us to differentiate between the correlated (elliptical) and uncorrelated energetic disorder). We found that the difference between the TA spectra corresponding to the absorption of one and two quanta suggests the presence of strong uncorrelated disorder (in agreement with earlier models of bacterial LH1/LH2 antennas). Nonlinear spectroscopy with multiple excitations may also be useful for exploring the exciton structure of other photosynthetic antennas and similar molecular systems.

Ultrafast electron transfer of different vibronic states in flavodoxin

Understanding the ultrafast electron transfer (ET) processes involving various vibronic excitation in biological systems remains challenging and requires well-defined model proteins for in-depth studies. In this work, we selected three types of mutants with different ET lifetimes in flavodoxin to explore the excitation dependence of the photo-induced ET dynamics. For the ultrafast ET processes within the time window of intramolecular vibrational relaxation, with increasing excitation energies, we observed no change of ET lifetimes but with higher vibrational excitation in products, both of which can be attributed to the unique nature of ultrafast ET. For the slower ET, we did not find any dependence of product vibration excitation relative to the initial different excitation due to the complete vibrational relaxation. These results strongly indicate the vibrational energy can be propagated to the subsequent reactions during ultrafast ET processes.

Deformation of Odd Nuclei 27Al, 31P and 35Cl in Single-Particle States

Using the evolutionary approach recently developed by us, the shapes of odd s-d-shell 27Al, 31P and 35Cl nuc­lei in the ground and single-particle excited states have been extracted from the experimental data on the energies, spins, and parities of these states, as well as the measured probabilities of electromagnetic transitions between them. The key ingredient of our procedure is the evolutionary algorithm that evolves the population of the bad-quality data-fitting nuclear shapes to the high-quality data-fitting nuclear shapes. We have found that the studied nuclei in the ground states are abnormally weakly deformed, which is not expected for the nuclei in the shell middle. Even in their low-laying single-particle excited states, the nuclei 27Al and 31P are found to be weakly deformed, too. With the increase of the single-particle excitation energy, the change of the state of the only one nucleon – the valence proton the spin and parity of which determine the spin and parity of the 35Cl nucleus – causes the shape phase transition from the high-symmetry phase – spherical ground state – to the low-symmetry phase – deformed excited states. The angular part of the 27Al and 31P nuclei shape is de­scribed by two harmonics – quadrupole and hexadecapole. The angular part of the 35Cl nucleus shape is de­scribed by three harmonics – quadrupole, hexadecapole, and hexacontatetrapole, but the contribution of hexadecapole deformation is not independent. At present, there are no fundamental nuclear models that account for or predict the dominant hexacontatetrapole deformation, especially for light and medium nuclei. We have found that the spin and parity of the 27Al, 31P and 35Cl nuc­lei are determined by the spin and parity of the last odd (valence) proton. At the same time, some of the nucleons of the nucleus core change their characteristics, too. Thus, the electromagnetic transitions between the single-particle states of the 27Al, 31P and 35Cl nuc­lei are the multi-particle processes.

Deformation dependence of thermal properties of hot rotating 184Re with inclusion of nuclear pairing fluctuations

In this work, the effects of nuclear deformation and angular momentum on the thermodynamic properties of 184Re have been investigated. In this regard, we have used the mean gap BCS (MEGBCS) model to consider thermal fluctuations. In this model, thermodynamic quantities are calculated using the mean gap parameter at various angular momenta. The results show that the pairing gap increases with increasing temperature at high angular momentum, known as thermally assisted pairing correlation. Also, the S-shaped heat capacity, interpreted as pairing phase transition, washes out with increasing angular momentum and nuclear deformation. We also calculated the nuclear level density versus excitation energy at various nuclear deformations and angular momenta. Compared with experimental data at J=(12±4)ħ, our results show that the shape of the nucleus changes as the excitation energy increases at J=12ħ. Also, our calculations on level density indicate that with increasing excitation energy at β2=0.23, the nuclear angular momentum increases from J=8ħ to J=16ħ.

Nuclear linear-chain structure arises in carbon-14

The shape and internal structure of an atomic nucleus can change significantly with increasing excitation energy, angular momentum, or isospin asymmetry. As an example of this structural evolution, linear-chain configurations in carbon or heavier isotopes have been predicted for decades. Recent studies have found non-stability of this structure in 12C while evidenced its appearance in 16C. It is then necessary to investigate the linear-chain molecular structures in 14C to clarify the exact location on the nuclear chart where this structure begins to emerge, and thus to benchmark theoretical models. Here we show a cluster-decay experiment for 14C with all final particles coincidentally detected, allowing a high Q-value resolution, and thus a clear decay-path selection. Unambiguous spin-parity analyses are conducted, strongly evidencing the emergence of the π-bond linear-chain molecular rotational band in 14C. The present results encourage further studies on even longer chain configurations in heavier neutron-rich nuclei.

Theoretical Study of Some Properties of High‐Temperature Two‐Band Model Iron‐Based Superconductor Ba1−xRbxFe2As2

This work focuses on the theoretical investigations of some properties of high‐temperature two‐band model iron‐based superconductor Ba1−xRbxFe2As2. The mathematical expressions of superconducting parameters are treated by the temperature‐dependent Green's function formalism. The superconducting order parameters of the material for electron, hole bands and for the interband are plotted as a function of temperature. The parameters decrease with increasing temperature and vanish at the superconducting transition temperature (Tc) of Ba1−xRbxFe2As2. The values of the superconducting order parameters and coupling strength in electron intraband, hole intraband, and the electron–hole interband are computed quantitatively. It is found that the pairing potential of Ba1−xRbxFe2As2 increases as a function of temperature. The phase diagrams of temperature‐dependent electron and hole intrabands and density of states versus excitation energy are plotted and it is observed that both decrease as the excitation energy increases. The impacts of temperature, pairing potential, and transitional temperature on condensation energy are also investigated and the results show that condensation energy decreases with the increase of temperature and pairing potential and vanishes at Tc of Ba1−xRbxFe2As2. Furthermore, the density of states is plotted for electron and hole‐intrabands at different temperatures versus excitation energy, and it is perceived that the density of states decreased gradually as the excitation energy increases and vanished at low‐temperature values. The current results are compatible with previous findings.

Energy-Dependent Charge Separation in Conjugated Polymer Electrolyte Complexes

Supramolecular complexes have great potential as light-harvesting materials, as intermolecular structural organization can be manipulated through steric or electrostatic interactions to impact electronic coupling between energy and charge donors and acceptors. Here, we examine the relative rates and efficiencies of charge transfer in conjugated polymer electrolyte complexes (CPECs) based on a polythiophene electron donor (PTAK) and polyfluorene electron acceptor (PFPI). These CPECs are characterized by ordered polymer microstructures, as evident from spectral signatures of strong excitonic coupling within the PTAK component from steady-state UV–vis absorption spectra. We find that PTAK polarons are generated within tens of picoseconds through a combination of prompt and delayed charge separation following direct photoexcitation or energy transfer from PFPI. Further, we find that decreasing the length of charged PTAK side chains or increasing excitation energy increases the driving force for electron transfer to increase charge separation rates and yields for polarons, with the greatest relative yields observed at excitation energies that initiate PFPI-to-PTAK energy transfer. Charge separation between components can be rationalized from a canonical Marcus picture, whereby excess vibrational energy effectively lowers the barrier for PTAK-to-PFPI charge separation. This contrasts with the recently reported ultrafast (<100 fs) charge separation in small-molecule/polythiophene electrolyte complexes that is attributed to strong orbital mixing that gives rise to charge generation via CT exciton states. These results provide insights into conditions for realizing charge separation in concert with energy transfer in CPECs as light-harvesting materials.

Semiconductor Bridge Temperature Change Study under Different Excitation Methods and Sizes

The temperature of the semiconductor bridge increases due to the Joule heat effect in the way of capacitive discharge voltage excitation and current excitation, but the temperature distribution and the effect of different excitation methods and different sizes on the temperature change are not very clear; aiming at this problem, the temperature distribution of the semiconductor bridge after electrical energy excitation is simulated by finite element simulation. The effects of excitation mode and bridge area size on the temperature rise process of the bridge were studied by the control variable method; finally, the high-temperature area of the bridge body is only distributed in the bridge area, the temperature of the bridge body is positively correlated with electrical energy but negatively correlated with the area of the bridge area, and the current excitation method is better than the effect of voltage excitation. In addition, the conclusion is that the appropriate increase in excitation energy and the reduction of bridge size can help the temperature rise to improve the efficiency of the semiconductor bridge in practical applications.

On the Energy-specific Photodissociation Pathways of 14N2 and 14N15N Isotopomers to N Atoms of Different Reactivity: A Quantum Dynamical Perspective

Photodissociation of the nitrogen molecule in the vacuum ultraviolet (VUV) is a major source of reactive nitrogen atoms in the upper atmosphere of Earth and throughout the solar system. Recent experimental studies have revealed strong energy dependence of the VUV photodissociation branching ratios to the N(4S3/2)+N(2D J ) and N(4S3/2)+N(2P J ) product channels, the primary dissociation pathways in the 108,000–116,000 cm−1 energy region. This produces N(2D J ) and N(2P J ) excited atoms that differ significantly in their chemical reactivity. The branching ratios oscillate with increase in the VUV excitation energy. We use high-level ab initio quantum chemistry to compute the potential curves of 17 electronic excited states and their nonadiabatic and spin–orbit couplings. The dynamics follow the sequential evolution from the optically excited but bound singlets. Spin–orbit coupling enables transfer to the dissociative triplet and quintet states. We compute the photodissociation yields through the dense manifold of electronic states leading to both exit channels. The dynamical simulations accurately capture the branching oscillations and enable a detailed look into the photodissociation mechanism. The major contribution to the dissociation is through the two lowest 3Π u states. However, for both isotopomers, at about 110,000 cm−1 there is an abnormally low dissociation rate into the N(4S3/2)+N(2P J ) channel that enables comparable participation of triplet and quintet 5Π u electronic states. This leads to the first peak in the branching ratio. At higher energies, trapping of the population in the 33Π u bound triplet state occurs. This favors dissociation to the lower-energy N(4S3/2)+N(2D J ) channel and results in the observed second switch in branching ratios.

Impact of the quenching of shell effects with excitation energy on nuclear level density

We investigate the nature and impact of shell effects on nuclear level density (NLD) and particle emission probability as a function of temperature in a microscopic theoretical framework of Statistical Model for nuclei ranging from neutron deficient to neutron rich isotopes of Z= 27−35. Critical temperatures are traced for neighbouring even, odd, closed shell and mid−shell nuclei which respond to excitations differently due to their varying stability and structural effects. Importance of the shell effects and shell correction energy is reflected significantly in NLD variation which slowly diminishes with increasing excitation energy indicating the quenching of shell effects. The enhancement of LD parameter with the deformation and rotation and the fade out of enhancement with increasing excitations has been shown. The weakening of magicity of N=28 near the proton drip line has been observed in the inverse level density parameter ‘K’(= A/a) and β variation though it is usually uncommon to see this effect in excited nuclei. Variation of our calculated NLD for odd 69As and even 70Ge exhibits structural effects and agrees very well with the variation of experimental values. Our evaluated level density parameter ‘a’ values are compared with RIPL-2 (Reference Input Parameter Library) data which show good agreement.

Influence of pre-scission neutron emission on high-energy U238 fission studied by the Langevin approach

Background: The information about the pre-scission emitted neutrons is crucial for the fission dynamics study such as fission time scale and dissipation mechanism. Purpose: The aim is to investigate the influence of the pre-scission neutron emission on the fragments mass, total kinetic energy, and the deformation distribution at the scission point in $^{238}\mathrm{U}(n,f)$ fissions at 110, 325, and 500 MeV. Method: A three-dimensional Langevin approach considering nucleus elongation, deformation, and mass asymmetry is applied to simulate fission dynamics. The Hauser-Feshbach statistical decay model is coupled to simulate the pre-scission neutron emission. Results: The properties of the pre-scission emitted neutron are first investigated. It is found that the multiplicity and the average kinetic energy increase with increasing excitation energy. The energy carried away by neutron emission is not negligible and it leads to the multichance fission, especially for the 110 MeV case. The fragments mass, total kinetic energy, and the deformation distribution at the scission point are then calculated with and without coupling the Hauser-Feshbach statistical decay model. It is found that the calculations partially reproduce the leftward shift of the experimental fragment mass distribution at high excitation energy. The pre-scission neutron emission also causes a reduction of the average total kinetic energy by about 2 MeV and a leftward shift of the distribution. The deformation distribution shows that about 75% of the fragments are produced in spherical and prolate ellipsoid shapes. No significant influence on deformation at the scission point was found. Conclusions: For high-energy fission, the energy reduction by pre-scission neutron emission should be considered, which will lead to the multichance fission. The pre-scission neutron emission is one of the reasons for the decrease of the average mass and the leftward shift of the fission fragments mass and total kinetic energy distribution.

Wavelength-Dependent Nonlinear Absorption in Platinum Nanoparticles at Off-Resonant Wavelength

In order to obtain optical nonlinear materials with high transparency and low propagation loss in the visible and infrared range, noble metal materials in the off-resonant band have become a hot spot in the optical field in recent years. Therefore, the nonlinear absorption characteristics of platinum nanoparticles (PtNPs) with the surface plasmon resonance (SPR) in the ultraviolet band were investigated with multi-wavelength (500–700 nm) nanosecond Z-scan technology. The measurement results showed that the SPR wavelength of PtNPs was far away from the excitation wavelength, but there were still the saturated absorption (SA) and the reverse saturated absorption (RSA) phenomena, and the size of nonlinear absorption was related to the excitation wavelength and the excitation energy. When the excitation wavelength was constant, with the increase in excitation energy, PtNPs converted from SA to RSA. When the excitation energy was constant, with the excitation wavelength approaching SPR, PtNPs also converted from SA to RSA. The SA and RSA phenomena in the off-resonant region were complementary to the systematic study of the nonlinearity of PtNPs.

Optical properties of mist CVD grown κ-Ga2O3

We report on the growth of crystalline κ-Ga2O3 using mist-chemical vapor deposition (CVD) on (0001) sapphire. The κ-phase was confirmed using high-resolution x-ray diffraction (XRD) θ-2θ scan and pole figure-scan of (122) reflex, respectively, while an on-axis full width at half maximum (FWHM) of 104 arcsec was measured in symmetric rocking curve scan. Further, the heteroepitaxial film was analyzed using a mosaic crystal model employing reciprocal space maps (RSM) and a series of asymmetric rocking curve scans. A bandgap of 5.1 eV and excitonic binding energy of 85 meV were estimated from absorption measurements. Built-in field, depletion width, and doping density level were extracted from parabolic WKB model fit. Cathodoluminescence (CL) spectroscopy revealed a defect peak at 329 nm, which was found to be blue-shift with increasing excitation energy, indicating a possible donor-acceptor pair (DAP) transition. Temperature-dependent Current–voltage (I–V) study was performed to extract Schottky barrier height () and ideality factor (η), which were found to be correlated with temperature. Photodetectors fabricated on the sample exhibited pure solar-blind characteristics.

Excitation-Energy-Dependent Photoluminescence Quantum Yield is Inherent to Optically Robust Core/Alloy-Shell Quantum Dots in a Vast Energy Landscape.

The importance of alloy-shelling in optically robust Core/Alloy-Shell (CAS) QDs has been described from structural and energetic aspects. Unlike fluorescent dyes, both Core/Shell (CS) and CAS QDs exhibit excitation-energy-dependent photoluminescence quantum yield (PLQY). For both CdSe and InP CAS QDs (with metal- and nonmetal-based alloy-shelling, respectively), with increasing excitation energy, (a) the ultrafast rise-time or relaxation-time to the band-edge increases and (b) the magnitude of the normalized bleach signal decreases. Ultrasensitive single-particle spectroscopic investigation results showed that with decreasing excitation energy, (a) the fraction of ON events increases, (b) the ratio of exciton-detrapping rate/trapping rate increases, and (c) the extent of beneficial hole trapping increases. A relative decrease in PLQY with increasing excitation energy is much less pronounced in CAS QDs than in CS QDs. Unless trap states are removed completely especially in the higher-energy landscape, PLQY will remain inherently dependent on excitation energy for QDs in the vast energy landscape. When reporting the PLQY of QDs, the magnitude of the excitation energy must be mentioned.

Fission of Hg*180,182,183 and Pt*178 nuclei at intermediate excitation energies

Background: The nature of asymmetric fission of preactinides is not yet understood in detail, despite intense experimental and theoretical studies carried out at present.Purpose: The study of asymmetric and symmetric fission of $^{180,182,183}\mathrm{Hg}$ and $^{178}\mathrm{Pt}$ nuclei as a function of their excitation energy and isospin.Methods: Mass-energy distributions of fission fragments of $^{180}\mathrm{Hg}, ^{178}\mathrm{Pt}$ (two protons less than $^{180}\mathrm{Hg}$), and $^{182}\mathrm{Hg}$ (two neutrons more than $^{180}\mathrm{Hg}$) formed in the $^{36}\mathrm{Ar}+^{144}\mathrm{Sm}, ^{142}\mathrm{Nd}$, and $^{40}\mathrm{Ca}+^{142}\mathrm{Nd}$ reactions were measured at energies near and above the Coulomb barrier. Fission of $^{183}\mathrm{Hg}$ obtained in the reaction of $^{40}\mathrm{Ca}$ with $^{143}\mathrm{Nd}$ was also investigated to see if one extra neutron could lead to dramatic changes in the fission process due to the shape-staggering effect in radii, known in $^{183}\mathrm{Hg}$.The measurements were performed with the double-arm time-of-flight spectrometer CORSET.Results: The observed peculiarities in the fission fragment mass-energy distributions for all studied nuclei may be explained by the presence of a symmetric fission mode and three asymmetric fission modes, manifested by the different total kinetic energies and fragment mass splits. The yield of symmetric mode grows with increasing excitation energy of compound nucleus.Conclusions: The investigated properties of asymmetric fission of $^{180,182,183}\mathrm{Hg}$ and $^{178}\mathrm{Pt}$ nuclei point out the existence of well-deformed proton shell at $Z\ensuremath{\approx}36$ and a less deformed proton shell at $Z$ \ensuremath{\approx} 46.

Dependence of total kinetic energy of fission fragments on the excitation energy of fissioning systems

We elucidated the reason why the average total kinetic energy (TKE) of fission fragments decreases when the excitation energy of the fissioning systems increase as indicated by experimental data for the neutron-induced fission events. To explore this problem, we used a method based on the four-dimensional Langevin equations we have developed. We have calculated the TKE of fission fragments for fissioning systems $^{236}\mathrm{U}^{*}$ and $^{240}\mathrm{Pu}^{*}$ excited above respective fission barriers, and compared the results with experimental data for n $+$ $^{235}\mathrm{U}$ and n $+$ $^{239}\mathrm{Pu}$ reactions, respectively. From the Langevin-model analysis, we have found that the shape of the abundant heavy fragments changes from almost spherical for low excitation domain to highly prolate shape for high excitation energy, while that of the light fragments does not change noticeably. The change of the ``shape'' of the heavy fragments causes an increase of a distance between the charge centers of the nascent fragments just after scission as excitation energy increases. Accordingly, the Coulomb repulsion between the two fragments decreases with an increase of the excitation energy, which causes the decrease of the average TKE. In this manner, we found that the change of the shape of the heavy fragment as a function of the excitation energy is the key issue for the TKE of fission fragments to decrease as the excitation energy of the fissioning nuclei increases. In other words, washing out of the shell effects, which affect the shape of the heavy fragments is the key reason for the decreasing energy dependence of the average TKE of the fission fragments.

Photoluminescence of InGaN-based red multiple quantum wells.

Optical properties of InGaN-based red LED structure, with a blue pre-well, are reported. Two emission peaks located at 445.1 nm (PB) and 617.9 nm (PR) are observed in the PL spectrum, which are induced by a low-In-content blue InGaN single quantum well (SQW) and the red InGaN double quantum wells (DQWs), respectively. The peak shift of PB with increase of excitation energy is very small, which reflects the built-in electric field of PB-related InGaN single QW is remarkably decreased, being attributed to the significant reduction of residual stress in the LED structure. On the other hand, the PR peak showed a larger shift with increase of excitation energy, due to both the screening of built-in electric field and the band filling effect. The electric field in the red wells is caused by the large lattice mismatch between high-In-content red-emitting InGaN and surrounding GaN. In addition, the anomalous temperature dependences of the PR peak are well elucidated by assuming that the red emission comes from quasi-QD structures with deep localized states. The deep localization suppresses efficiently the escape of carriers and then enhances the emission in the red, leading to high internal quantum efficiency (IQE) of 24.03%.

Photovoltaic distribution on an amorphous-silicon solar cell in near-band-edge excitation observed by conductive-probe atomic force microscopy combined with a fine-wavelength-tunable light source.

We describe the development of a conductive-probe atomic force microscopy method combined with a fine-wavelength-tunable light source and use it to observe the photovoltaic distribution on a cross-sectional surface of an amorphous-silicon solar cell in near-band-edge excitation. The light source's wavelength resolution is dλ = 1nm, and its intensity is 1 µW/cm2 (10 mW/m2); this excitation condition is sufficiently fine and weak to investigate electrical properties in the near-band-edge wavelength range. The photovoltage is observed in the indium tin oxide (ITO) region, and the maximum photovoltage increases when we increase the excitation energy of the illumination light. However, the photovoltaic distribution parallel to the ITO layer becomes relatively localized as the excitation energy increases. This localized photovoltaic distribution suggests that the conductivity of the electric current path within the ITO layer should be inhomogeneous.

Dimethylcarbene versus Direct Propene Formation in Dimethylketene Photodissociation.

Highly reactive carbenes are usually produced by photolysis of ketenes, diazoalkanes, or diazirines. Sequential kinetic pathways for deactivation of nascent carbenes usually involve bimolecular reactions in competition with isomerization producing stable products such as alkenes. However, the direct photolytic production of stable products, effectively bypassing formation of free carbenes, has been postulated for over 50 years but remains very poorly understood. Often termed "rearrangement in the excited state" (RIES), examples include 1,2-hydrogen migration within photoexcited carbene precursors yielding alkenes and the Wolff rearrangement in photogenerated carbonyl-substituted carbenes producing ketenes. In this study, the two competing CO elimination channels from photoexcited gaseous dimethylketene, producing dimethylcarbene and propene, were studied as a function of electronic excitation energy, under collision-free conditions, by using photofragment translational energy spectroscopy with vacuum ultraviolet photoionization of the products. A significant fraction of the dimethylcarbene → propene isomerization exothermicity (∼300 kJ/mol) was released as propene + CO translational energy, indicating that propene is formed prior to or concurrent with CO elimination. An increase in the propene yield with increasing excitation energy suggests that the effective potential energy barrier for this channel lies ∼24 kJ/mol above the energetic threshold for dimethylcarbene formation via C═C bond fission. Possible mechanisms for direct propene elimination are discussed in light of the observed energy dependence for the competing pathways.