Photodissociation Rates Research Articles

The Impact of UV Radiation on Circumstellar Chemistry

AbstractUV radiation plays a critical role in the chemistry of circumstellar envelopes (CSEs) around evolved stars on the asymptotic giant branch (AGB). However, the effects of all potential sources of UV radiation have not been included in models. We present preliminary results of adding an internal source of UV to the CSE chemistry and predict large enhancements of atomic and ionic species arising from photo-destruction of parent species. Observations of atomic carbon towards the UV-bright AGB star o Cet are consistent with the modelling. In addition, we calculate the precise depth dependence of the CO photodissociation rate in an expanding CSE. We incorporate this within a chemical network active in the outflows of AGB stars, which includes 933 species and 15182 reactions. Our results show that the CO envelope size is about 30% smaller at half abundance than the most commonly used radius reported by Mamon et al. (1988).

The chemistry of disks around T Tauri and Herbig Ae/Be stars.

Infrared and (sub-)mm observations of disks around T Tauri and Herbig Ae/Be stars point to a chemical differentiation between both types of disks, with a lower detection rate of molecules in disks around hotter stars. To investigate the underlying causes of the chemical differentiation indicated by observations we perform a comparative study of the chemistry of T Tauri and Herbig Ae/Be disks. This is one of the first studies to compare chemistry in the outer regions of these two types of disks. We developed a model to compute the chemical composition of a generic protoplanetary disk, with particular attention to the photochemistry, and applied it to a T Tauri and a Herbig Ae/Be disk. We compiled cross sections and computed photodissociation and photoionization rates at each location in the disk by solving the FUV radiative transfer in a 1+1D approach using the Meudon PDR code and adopting observed stellar spectra. The warmer disk temperatures and higher ultraviolet flux of Herbig stars compared to T Tauri stars induce some differences in the disk chemistry. In the hot inner regions, H2O, and simple organic molecules like C2H2, HCN, and CH4 are predicted to be very abundant in T Tauri disks and even more in Herbig Ae/Be disks, in contrast with infrared observations that find a much lower detection rate of water and simple organics toward disks around hotter stars. In the outer regions, the model indicates that the molecules typically observed in disks, like HCN, CN, C2H, H2CO, CS, SO, and HCO+, do not have drastic abundance differences between T Tauri and Herbig Ae disks. Some species produced under the action of photochemistry, like C2H and CN, are predicted to have slightly lower abundances around Herbig Ae stars due to a narrowing of the photochemically active layer. Observations indeed suggest that these radicals are somewhat less abundant in Herbig Ae disks, although in any case the inferred abundance differences are small, of a factor of a few at most. A clear chemical differentiation between both types of disks concerns ices. Owing to the warmer temperatures of Herbig Ae disks, one expects snowlines lying farther away from the star and a lower mass of ices compared to T Tauri disks. The global chemical behavior of T Tauri and Herbig Ae/Be disks is quite similar. The main differences are driven by the warmer temperatures of the latter, which result in a larger reservoir or water and simple organics in the inner regions and a lower mass of ices in the outer disk.

The efficiency of photodissociation for molecules in interstellar ices

Processing by interstellar photons affects the composition of the icy mantles on interstellar grains. The rate of photodissociation in solids differs from that of molecules in the gas phase. The aim of this work was to determine an average, general ratio between photodissociation coefficients for molecules in ice and gas. A 1D astrochemical model was utilized to simulate the chemical composition for a line of sight through a collapsing interstellar cloud core, whose interstellar extinction changes with time. At different extinctions, the calculated column densities of icy carbon oxides and ammonia (relative to water ice) were compared to observations. The latter were taken from literature data of background stars sampling ices in molecular clouds. The best-fit value for the solid/gas photodissociation coefficient ratio was found to be ~0.3. In other words, gas-phase photodissociation rate coefficients have to be reduced by a factor of 0.3 before applying them to icy species. A crucial part of the model is a proper inclusion of cosmic-ray induced desorption. Observations sampling gas with total extinctions in excess of ~22 mag were found to be uncorrelated to modelling results, possibly because of grains being covered with non-polar molecules.

Photodissociation of CS from Excited Rovibrational Levels

Abstract Accurate photodissociation cross sections have been computed for transitions from the X 1Σ+ ground electronic state of CS to six low-lying excited electronic states. New ab initio potential curves and transition dipole moment functions have been obtained for these computations using the multi-reference configuration interaction approach with the Davidson correction (MRCI+Q) and aug-cc-pV6Z basis sets. State-resolved cross sections have been computed for transitions from nearly the full range of rovibrational levels of the X 1Σ+ state and for photon wavelengths ranging from 500 Å to threshold. Destruction of CS via predissociation in highly excited electronic states originating from the rovibrational ground state is found to be unimportant. Photodissociation cross sections are presented for temperatures in the range between 1000 and 10,000 K, where a Boltzmann distribution of initial rovibrational levels is assumed. Applications of the current computations to various astrophysical environments are briefly discussed focusing on photodissociation rates due to the standard interstellar and blackbody radiation fields.

Collective effect of light-induced and natural nonadiabatic phenomena in the dissociation dynamics of the NaI molecule

Natural and light-induced nonadiabatic effects are ubiquitous in many photochemical and photophysical processes. Here we study the interplay between them when they are present simultaneously in a molecular system. Our showcase example is the NaI molecule. Solving the time-dependent nuclear Schrödinger equation the photodissociation rate and the angular distribution of the molecular photofragments are calculated and discussed at several resonant laser energies and intensities. Obtained results clearly demonstrate that the dissociation rate and the angular distribution of the NaI photofragments can be considered as clear fingerprint of a collective effect of light-induced and natural nonadiabatic phenomena.

VUV-absorption cross section of carbon dioxide from 150 to 800 K and applications to warm exoplanetary atmospheres

Context. Most exoplanets detected so far have atmospheric temperatures significantly higher than 300 K. Often close to their star, they receive an intense UV photons flux that triggers important photodissociation processes. The temperature dependency of vacuum ultraviolet (VUV) absorption cross sections are poorly known, leading to an undefined uncertainty in atmospheric models. Similarly, data measured at low temperatures similar to those of the high atmosphere of Mars, Venus, and Titan are often lacking. Aims. Our aim is to quantify the temperature dependency of the VUV absorption cross sections of important molecules in planetary atmospheres. We want to provide high-resolution data at temperatures prevailing in these media, and a simple parameterisation of the absorption in order to simplify its use in photochemical models. This study focuses on carbon dioxide (CO2). Methods. We performed experimental measurements of CO2 absorption cross sections with synchrotron radiation for the wavelength range (115–200 nm). For longer wavelengths (195–230 nm), we used a deuterium lamp and a 1.5 m Jobin-Yvon spectrometer. We used these data in our one-dimensional (1D) thermo-photochemical model in order to study their impact on the predicted atmospheric compositions. Results. The VUV absorption cross section of CO2 increases with the temperature. The absorption we measured at 150 K seems to be close to the absorption of CO2 in the fundamental ground state. The absorption cross section can be separated into two parts: a continuum and a fine structure superimposed on the continuum. The variation in the continuum of absorption can be represented by the sum of three Gaussian functions. Using data at high temperature in thermo-photochemical models significantly modifies the abundance and the photodissociation rates of many species in addition to CO2, such as methane and ammonia. These deviations have an impact on synthetic transmission spectra, leading to variations of up to 5 ppm. Conclusions. We present a full set of high-resolution (Δλ = 0.03 nm) absorption cross sections of CO2 from 115 to 230 nm for temperatures ranging from 150 to 800 K. A parameterisation allows us to calculate the continuum of absorption in this wavelength range. Extrapolation at higher temperature has not been validated experimentally and therefore should be used with caution. Similar studies on other major species are necessary to improve our understanding of planetary atmospheres.

Water vapor in the middle atmosphere of Mars during the 2007 global dust storm

Recent observations of the Martian hydrogen corona in the UV H Ly-alpha emission by the Hubble Space Telescope (HST) (Clarke et al., 2014) and the SPICAM UV spectrometer on Mars Express (Chaffin et al., 2014) reported its rapid change by an order of magnitude over a short few months period in 2007 (MY28), which is inconsistent with the existing models. One proposed explanation of the observed increase of the coronal emission is that during the global dust storm water vapor from the lower atmosphere can be transported to higher altitudes, where its photodissociation rate by near-UV sunlight increases, providing an additional source of hydrogen for the upper atmosphere.In this work we study the water vapor vertical distribution in the middle atmosphere of Mars during the 2007 global dust storm based on solar occultation measurements by the SPICAM IR spectrometer onboard the Mars-Express spacecraft. The vertical profiles of H2O density and mixing ratio have been obtained for solar longitudes Ls = 255°–300° in MY28. In the Northern hemisphere from Ls = 268° to Ls = 285° the H2O density at altitudes of 60–80 km increased by an order of magnitude. During the dust storm the profiles extended up to 80 km, with an H2O density exceeding 1010 molecules/cm3 (mixing ratio ≥200 ppm). Two maxima of the H2O density were detected. The largest H2O densities observed at latitudes higher than 60°N, over Ls = 269°–275°, do not directly correlate with the aerosol loading and likely relate to the downwelling branch of the meridional circulation that was intensified during the dust storm, and transported water from the Southern hemisphere to high northern latitudes. The second smaller maximum coincides with the high dust loading at middle northern latitudes. The comparison with geographically close observations in the quiet Mars year MY32, when the H2O content in the Northern hemisphere did not exceed 2 × 1010 molecules/cm3 and 50 ppm at 60 km, showed that the global dust storm was a unique event. The situation was different in the Southern hemisphere. During the dust storm the water density at 50–80 km increased by a factor of 4–5 with a mixing ratio >100 ppm, well correlated with the aerosol vertical extension. A somewhat weaker increase of the H2O density by a factor of 2–3 with a mixing ratio > 100 ppm was also observed during MY32 starting from Ls = 260°, suggesting a seasonal repeatability.The observed amount of water at high altitudes in both hemispheres can produce a large increase in the H escape rate on a timescale of weeks, as was shown in the photochemical modeling by Chaffin et al. (2017). Future modeling would be necessary to separate the seasonal and the dust storm contributions to the hydrogen escape.

Ruthenium Complexes are pH-Activated Metallo Prodrugs (pHAMPs) with Light-Triggered Selective Toxicity Toward Cancer Cells.

Metallo prodrugs that take advantage of the inherent acidity surrounding cancer cells have yet to be developed. We report a new class of pH-activated metallo prodrugs (pHAMPs) that are activated by light- and pH-triggered ligand dissociation. These ruthenium complexes take advantage of a key characteristic of cancer cells and hypoxic solid tumors (acidity) that can be exploited to lessen the side effects of chemotherapy. Five ruthenium complexes of the type [(N,N)2Ru(PL)]2+ were synthesized, fully characterized, and tested for cytotoxicity in cell culture (1A: N,N = 2,2'-bipyridine (bipy) and PL, the photolabile ligand, = 6,6'-dihydroxybipyridine (6,6'-dhbp); 2A: N,N = 1,10-phenanthroline (phen) and PL = 6,6'-dhbp; 3A: N,N = 2,3-dihydro-[1,4]dioxino[2,3-f][1,10]phenanthroline (dop) and PL = 6,6'-dhbp; 4A: N,N = bipy and PL = 4,4'-dimethyl-6,6'-dihydroxybipyridine (dmdhbp); 5A: N,N = 1,10-phenanthroline (phen) and PL = 4,4'-dihydroxybipyridine (4,4'-dhbp). The thermodynamic acidity of these complexes was measured in terms of two pKa values for conversion from the acidic form (XA) to the basic form (XB) by removal of two protons. Single-crystal X-ray diffraction data is discussed for 2A, 2B, 3A, 4B, and 5A. All complexes except 5A showed measurable photodissociation with blue light (λ = 450 nm). For complexes 1A-4A and their deprotonated analogues (1B-4B), the protonated form (at pH 5) consistently gave faster rates of photodissociation and larger quantum yields for the photoproduct, [(N,N)2Ru(H2O)2]2+. This shows that low pH can lead to greater rates of photodissociation. Cytotoxicity studies with 1A-5A showed that complex 3A is the most cytotoxic complex of this series with IC50 values as low as 4 μM (with blue light) versus two breast cancer cell lines. Complex 3A is also selectively cytotoxic, with sevenfold higher toxicity toward cancerous versus normal breast cells. Phototoxicity indices with 3A were as high as 120, which shows that dark toxicity is avoided. The key difference between complex 3A and the other complexes tested appears to be higher uptake of the complex as measured by inductively coupled plasma mass spectrometry, and a more hydrophobic complex as compared to 1A, which may enhance uptake. These complexes demonstrate proof of concept for dual activation by both low pH and blue light, thus establishing that a pHAMP approach can be used for selective targeting of cancer cells.

Non-thermal photons and direct photodissociation of H2, HD and HeH+ in the chemistry of the primordial Universe

Non-thermal photons deriving from radiative transitions among the internal ladder of atoms and molecules are an important source of photons in addition to thermal and stellar sources in many astrophysical environments. In the present work the calculation of reaction rates for the direct photodissociation of some molecules relevant in early Universe chemistry is presented; in particular, the calculations include non-thermal photons deriving from the recombination of primordial hydrogen and helium atoms for the cases of H2, HD and HeH+. New effects on the fractional abundances of chemical species are investigated and the fits for the HeH+ photodissociation rates by thermal photons are provided.

Photodissociation and photoionisation of atoms and molecules of astrophysical interest

A new collection of photodissociation and photoionisation cross sections for 102 atoms and molecules of astrochemical interest has been assembled, along with a brief review of the basic processes involved. These have been used to calculate dissociation and ionisation rates, with uncertainties, in a standard ultraviolet interstellar radiation field (ISRF) and wavelength-dependent radiation fields. The new ISRF rates generally agree within 30% with our previous compilations, with a few notable exceptions. The reduction of rates in shielded regions was calculated as a function of dust, molecular and atomic hydrogen, atomic C, and self-shielding column densities. The relative importance of shielding types depends on the species in question and the dust optical properties. The new data are publicly available from the Leiden photodissociation and ionisation database. Sensitivity of rates to variation of temperature and isotope, and cross section uncertainties, are tested. Tests were conducted with an interstellar-cloud chemical model, and find general agreement (within a factor of two) with the previous iteration of the Leiden database for the ISRF, and order-of-magnitude variations assuming various kinds of stellar radiation. The newly parameterised dust-shielding factors makes a factor-of-two difference to many atomic and molecular abundances relative to parameters currently in the UDfA and KIDA astrochemical reaction databases. The newly-calculated cosmic-ray induced photodissociation and ionisation rates differ from current standard values up to a factor of 5. Under high temperature and cosmic-ray-flux conditions the new rates alter the equilibrium abundances of abundant dark cloud abundances by up to a factor of two. The partial cross sections for H2O and NH3 photodissociation forming OH, O, NH2 and NH are also evaluated and lead to radiation-field-dependent branching ratios.

Speed of flame propagation in CH3Cl + Cl2 mixtures during combustion initiated by continuous UV radiation

Flame propagation in a closed tube over mixtures of chloromethane and chlorine of different compositions following ignition by continuous UV radiation is studied. It is found that the rate of combustion in all mixtures except limiting ones grows along with the propagation of the flame front up to its maximum values at nearly 1/3 the tube length and then slows. In limiting mixtures, the speed’s behavior is completely different. It is greatest near the source of UV light and gradually slows with distance from the source. The high speed in the initial section is due to the effect of UV light. The temperature of combustion is lowest in limiting mixtures, and the rate of chlorine molecule photodissociation at this temperature is comparable to and even faster than that of their thermal dissociation. The light in these mixtures thus contributes substantially to the initiation of the chemical reaction. It is concluded that when limiting mixtures are ignited by UV pulses, the speed of flame propagation falls markedly as it proceeds without the influence of radiation, and the character of changes in the speed’s behavior becomes identical to those for other mixtures.

The Photodissociation of HCN and HNC: Effects on the HNC/HCN Abundance Ratio in the Interstellar Medium.

The impact of the photodissociation of HCN and HNC isomers is analyzed in different astrophysical environments. For this purpose, the individual photodissociation cross section of HCN and HNC isomers have been calculated in the 7-13.6 eV photon energy range for a temperature of 10 K. These calculations are based on the ab initio calculation of three-dimensional adiabatic potential energy surfaces of the 21 lower electronic states. The cross sections are then obtained using a quantum wave packet calculation of the rotational transitions needed to simulate a rotational temperature of 10 K. The cross section calculated for HCN shows significant differences with respect to the experimental one, and this is attributed to the need of considering non-adiabatic transitions. Ratios between the photodissociation rates of HCN and HNC under different ultraviolet radiation fields have been computed by renormalizing the rates to the experimental one. It is found that HNC is photodissociated faster than HCN by a factor of 2.2, for the local interstellar radiation field, and 9.2, for the solar radiation field at 1 au. We conclude that to properly describe the HNC/HCN abundance ratio in astronomical environments illuminated by an intense ultraviolet radiation field it is necessary to use different photodissociation rates for each of the two isomers, obtained by integrating the product of the photodissociation cross sections and ultraviolet radiation field over the relevant wavelength range.

CHEMISTRY IN A FORMING PROTOPLANETARY DISK: MAIN ACCRETION PHASE

ABSTRACT We investigate the chemistry in a radiation-hydrodynamics model of a star-forming core that evolves from a cold (∼10 K) prestellar core to the main accretion phase in ∼105 years. A rotationally supported gravitationally unstable disk is formed around a protostar. We extract the temporal variation of physical parameters in ∼1.5 × 103 SPH particles that end up in the disk, and perform post-processing calculations of the gas-grain chemistry adopting a three-phase model. Inside the disk, the SPH particles migrate both inward and outward. Since a significant fraction of volatiles such as CO can be trapped in the water-dominant ice in the three-phase model, the ice mantle composition depends not only on the current position in the disk, but also on whether the dust grain has ever experienced higher temperatures than the water sublimation temperature. Stable molecules such as H2O, CH4, NH3, and CH3OH are already abundant at the onset of gravitational collapse and are simply sublimated as the fluid parcels migrate inside the water snow line. On the other hand, various molecules such as carbon chains and complex organic molecules (COMs) are formed in the disk. The COMs abundance sensitively depends on the outcomes of photodissociation and diffusion rates of photofragments in bulk ice mantle. As for S-bearing species, H2S ice is abundant in the collapse phase. In the warm regions in the disk, H2S is sublimated to be destroyed, while SO, H2CS, OCS, and SO2 become abundant.

Quadrupole association and dissociation of hydrogen in the early Universe

Radiative association and photodissociation rates are calculated for quadrupole transitions of H2. A complete set of bound and unbound states are included in a self-consistent master equation to obtain steady-state concentrations for a dilute system of hydrogen atoms and molecules. Phenomenological rate constants computed from the steady-state concentrations satisfy detailed balance for any combination of matter and radiation temperature. Simple formulas are derived for expressing the steady-state distributions in terms of equilibrium distributions. The rate constant for radiative association is found to be generally small for all temperature combinations. The photodissociation rate constant for quadrupole transitions is found to dominate the rate constants for other H2 photodestruction mechanisms for 3000 K. Implications for the formation and destruction of H2 in the early Universe are discussed.

THE MUSCLES TREASURY SURVEY. III. X-RAY TO INFRARED SPECTRA OF 11 M AND K STARS HOSTING PLANETS

ABSTRACT We present a catalog of panchromatic spectral energy distributions (SEDs) for 7 M and 4 K dwarf stars that span X-ray to infrared wavelengths (5 Å –5.5 μm). These SEDs are composites of Chandra or XMM-Newton data from 5–∼50 Å, a plasma emission model from ∼50–100 Å, broadband empirical estimates from 100–1170 Å, Hubble Space Telescope data from 1170–5700 Å, including a reconstruction of stellar Lyα emission at 1215.67 Å, and a PHOENIX model spectrum from 5700–55000 Å. Using these SEDs, we computed the photodissociation rates of several molecules prevalent in planetary atmospheres when exposed to each star’s unattenuated flux (“unshielded” photodissociation rates) and found that rates differ among stars by over an order of magnitude for most molecules. In general, the same spectral regions drive unshielded photodissociations both for the minimally and maximally FUV active stars. However, for O3 visible flux drives dissociation for the M stars whereas near-UV flux drives dissociation for the K stars. We also searched for an far-UV continuum in the assembled SEDs and detected it in 5/11 stars, where it contributes around 10% of the flux in the range spanned by the continuum bands. An ultraviolet continuum shape is resolved for the star ϵ Eri that shows an edge likely attributable to Si ii recombination. The 11 SEDs presented in this paper, available online through the Mikulski Archive for Space Telescopes, will be valuable for vetting stellar upper-atmosphere emission models and simulating photochemistry in exoplanet atmospheres.

THE ROLE OF NITROGEN IN TITAN’S UPPER ATMOSPHERIC HYDROCARBON CHEMISTRY OVER THE SOLAR CYCLE

ABSTRACT Titan’s thermospheric photochemistry is primarily driven by solar radiation. Similarly to other planetary atmospheres, such as Mars’, Titan’s atmospheric structure is also directly affected by variations in the solar extreme-UV/UV output in response to the 11-year-long solar cycle. Here, we investigate the influence of nitrogen on the vertical production, loss, and abundance profiles of hydrocarbons as a function of the solar cycle. Our results show that changes in the atmospheric nitrogen atomic density (primarily in its ground state N(4S)) as a result of photon flux variations have important implications for the production of several minor hydrocarbons. The solar minimum enhancement of CH3, C2H6, and C3H8, despite the lower CH4 photodissociation rates compared with solar maximum conditions, is explained by the role of N(4S). N(4S) indirectly controls the altitude of termolecular versus bimolecular chemical regimes through its relationship with CH3. When in higher abundance during solar maximum at lower altitudes, N(4S) increases the importance of bimolecular CH3 + N(4S) reactions producing HCN and H2CN. The subsequent remarkable CH3 loss and decrease in the CH3 abundance at lower altitudes during solar maximum affects the overall hydrocarbon chemistry.

Effect of photochemical self-action of carbon-containing aerosol: Wildfires

It has been shown by numerical simulation that the rate of formation of secondary organic aerosols (SOAs) in smoke plumes caused by vegetation and peat fires under real conditions can significantly depend on the aerosol optical thickness (AOT). The AOT determines the photodissociation rate and hydroxyl radical concentration, which in turn determines the rate of SOA generation as a result of oxidation of semivolatile organic compounds. Quantitative analysis has been carried out for the situation that took place in European Russia during the 2010 Russian wildfires. The state-of-the-art 3D chemical transport model is used in this study; the simulations are optimized and validated using the data of monitoring of the particulate matter in the Moscow region and Finland. The findings indicate that it is important to allow for this effect in studies focused on the analysis and prediction of air pollution due to wildfires, as well as climate and weather studies, whose results may depend on the assumptions about the content and properties of atmospheric carbon-containing aerosol.

Photodissociation of HCN and HNC isomers in the 7-10 eV energy range.

The ultraviolet photoabsorption spectra of the HCN and HNC isomers have been simulated in the 7-10 eV photon energy range. For this purpose, the three-dimensional adiabatic potential energy surfaces of the 7 lowest electronic states, and the corresponding transition dipole moments, have been calculated, at multireference configuration interaction level. The spectra are calculated with a quantum wave packet method on these adiabatic potential energy surfaces. The spectra for the 3 lower excited states, the dissociative electronic states, correspond essentially to predissociation peaks, most of them through tunneling on the same adiabatic state. The 3 higher electronic states are bound, hereafter electronic bound states, and their spectra consist of delta lines, in the adiabatic approximation. The radiative lifetime towards the ground electronic states of these bound states has been calculated, being longer than 10 ns in all cases, much longer that the characteristic predissociation lifetimes. The spectra of HCN is compared with the available experimental and previous theoretical simulations, while in the case of HNC there are no previous studies to our knowledge. The spectrum for HNC is considerably more intense than that of HCN in the 7-10 eV photon energy range, which points to a higher photodissociation rate for HNC, compared to HCN, in astrophysical environments illuminated by ultraviolet radiation.

Impact of Nuclear Mass Uncertainties on the r Process.

Nuclear masses play a fundamental role in understanding how the heaviest elements in the Universe are created in the r process. We predict r-process nucleosynthesis yields using neutron capture and photodissociation rates that are based on the nuclear density functional theory. Using six Skyrme energy density functionals based on different optimization protocols, we determine for the first time systematic uncertainty bands-related to mass modeling-for r-process abundances in realistic astrophysical scenarios. We find that features of the underlying microphysics make an imprint on abundances especially in the vicinity of neutron shell closures: Abundance peaks and troughs are reflected in trends of neutron separation energy. Further advances in the nuclear theory and experiments, when linked to observations, will help in the understanding of astrophysical conditions in extreme r-process sites.

A Reversibly Photodissociable Split GFP

Photocaged molecules, including split Green Fluorescent Protein (GFP), allow for light-activation of in vivo processes or cargo delivery in a minimally invasive manner while offering precise spatial and temporal resolution. GFP has been circularly permuted to have an N-terminal β-strand (strand 7, s7) with a protease cleavage site inserted in a loop added between s7 and the rest of the protein. After cleavage of the loop to create the split protein, the s7 peptide does not dissociate measurably from GFP. In the presence of light, however, s7 photodissociates from the rest of the protein, resulting in a change in UV-Vis absorbance and diminished fluorescence, similar to a previously reported photodissociable split GFP involving β-strand 10 [1]. The rate of photodissociation depends linearly on the number of incident photons until a light-saturated regime is reached, indicating that photodissociation is a single-photon process. In addition, the rate is a function of temperature, indicating that the rate-limiting step is a thermal dissociation step. Re-association with the missing strand occurs spontaneously without the addition of light to form GFP with spectral properties indistinguishable from the original protein. While split GFP has previously been designed for use as a genetically encoded fluorescence reporter of protein-protein interactions [2], this new s7 split GFP represents the next generation of photocaged molecules, creating the opportunity for active control of protein-protein interactions with light.1. Do, K.; Boxer, S. G., J. Am. Chem. Soc., 2011,133, 18078–18081.2. Cabantous et al., Scientific Reports, 2013, 3, 2854.