Volume 92, Issue 2, 15 January 1990
Index of content:
Evolution of molecular dissociation through an electronic curve crossing: Polarized emission spectroscopy of CH3I at 266 nm92(1990); http://dx.doi.org/10.1063/1.458116View Description Hide Description
We present a study of the dissociation of CH3I on coupled repulsive electronic potential energy surfaces by the technique of polarized emission spectroscopy. We excite CH3I at 266 nm and disperse the photons emitted from the dissociating molecule by both frequency and angular distribution with respect to the polarization direction of the excitation laser. We thus measure the polarization of the first 12 C–I stretching emission features, corresponding to the spectral region between 266 and 317 nm. We also obtain the rotational envelope of selected emission features in higher resolution scans and model the lineshapes with parameters derived from the polarization results. The polarization measurements show the emission into the first few low‐lying C–I stretching vibrational levels is via a transition moment parallel to the absorbing one, consistent with excitation to and emission from the 3 Q 0(2A 1) repulsive surface. Emission to higher C–I stretching overtones shows an increasing contribution from emission via a transition moment perpendicular to the absorbing one, consistent with emission from a repulsive surface of E symmetry following excitation to the 3 Q 0(2A 1) state.
We extract from the data the fraction of photons emitted via a perpendicular transition for each of the C–I stretch emission features. The analysis includes the derivation of analytic expressions for the angular distribution of the photons, with and without integration over the rotational contour, when the detector has a finite acceptance angle. We discuss the results in relation to a simple model where photoabsorption excites the molecule to the 3 Q 0(2A 1) repulsive surface (parallel transition moment) and amplitude develops on the 1 Q 1(3E) repulsive surface as the molecule dissociates through a curve crossing. The changes in amplitude of the molecular wavefunction on the A 1 vs the E repulsive surfaces during dissociation is thus probed. We outline a crude classical quasidiatomic approximation for roughly extracting from our data the electronic energy at which the ‘‘curve crossing’’ occurs. This derived energy is compared to that given in model and a b i n i t i o calculations of the excited electronic potential energy surfaces. Finally, we discuss the results in relation to the simple quasidiatomic Landau–Zener crossing model utilized by other workers, a model which does not fully explain the collection of experimental results over the last decade on the iodoalkane curve crossing.
Predissociation linewidths of the (1,0)–(12,0) Schumann–Runge absorption bands of O2 in the wavelength region 179–202 nm92(1990); http://dx.doi.org/10.1063/1.458117View Description Hide Description
A nonlinear least‐squares method of retrieving predissociationlinewidths from the experimental absolute absorption cross sections of Yoshino e t a l. [Planet. Space Sci. 3 1, 339 (1983)] has been applied to the (1,0)–(12,0) Schumann–Runge bands of oxygen. Predissociationlinewidths deduced for the Schumann–Runge bands are larger than the theoretical predictions of Julienne [J. Mol. Spectrosc. 6 3, 60 (1976)] and the latest measurements of Lewis e t a l. [J. Quant. Spectrosc. Radiat. Transfer 3 6, 187 (1986)]. The larger linewidths found will have an impact on calculations of solar flux penetration into the Earth’s atmosphere and of the photodissociation rates of trace species in the upper atmosphere. Systematic variation of predissociationlinewidths with rotational quantum number is observed in the bands (v’,0) with v’=6, 8, 9, 11, and 12.
92(1990); http://dx.doi.org/10.1063/1.458589View Description Hide Description
The absorptionspectrum of a supersonic free jet expansion of 1 4N2 in He has been photographically recorded in the region of the He continuum from ∼900 to 650 Å at reciprocal dispersions of 0.26 and 0.19 Å/mm. The low rotational temperature of close to 20 K results in a drastic simplification of the band structures compared to room or even liquid nitrogen temperature spectra. This report concentrates on the region from 835 to 790 Å and presents a survey of vibronic structures up to the first ionization potential. The Rydberg series of n p complexes converging to v=0 in the ground stateX 2Σ+ g of the ion can be followed to n=39, the low‐J rotational structure being clearly discernible in complexes as high as n=19. The application of multichannel quantum defect theory to a least‐squares analysis of four series of rovibronic levels associated with the lowest rotational levels N +=0, 1, and 2 of the ion leads to very substantially reduced error limits for the ionization potential of 1 4N2 and reveals a number of irregularities that point to perturbations by Rydberg levels derived from A 2Π u of N+ 2.
92(1990); http://dx.doi.org/10.1063/1.458118View Description Hide Description
Microwave rotational transitions have been observed for HCN–(CO2)3, DCN–(CO2)3, H1 3CN–(CO2)3, HC1 5N–(CO2)3, HCN–(1 3CO2)3, HCN–(1 8OCO)(CO2)2, and HCN–(CO2)(C1 8O2)2 with the pulsed Fourier transform, Flygare/Balle Mark II spectrometer. A symmetric top spectrum was observed for the parent isotopic species with rotational constants of B 0=861.6392(1) MHz, D J =0.681(5) kHz, and D J K =0.821(12) kHz. The results for isotopic substitution indicate a zero‐point, vibrationally averaged geometry having the C 3 symmetry of a cyclic (CO2)3 structure with the HCN along the symmetry axis and the N end closest to the (CO2)3. The C 3 symmetry is confirmed by the observation of states limited to K=±3n, with n=0,1,2,..., as predicted for threefold symmetry generated by bosons only. The (CO2)3 has a pinwheel configuration, as in the free trimer, and the three carbons lie in a plane R=2.758 Å below the center of mass (c.m.) of the HCN. The C‐C distance in this subunit is 3.797 Å which is 0.241 Å shorter than that found in the free (CO2)3 trimer. The individual CO2’s in the (CO2)3 pinwheel are rotated out of the C–C–C plane by γ=−6.9°, as determined from an inertial analysis, with the inner oxygens rotated away from the HCN. The HCN has an average torsional angle of 10.3°, as determined by isotopic substitution, and an observed χ c c value of −3.891 MHz for the 14 N. The c.m.(HCN) to C distance is 3.525 Å, compared to 3.592 Å in the HCN‐CO2 T‐shaped dimer. The isotopic substitution by 1 8O perturbs the structure of the symmetric top clusters by a remarkable amount, decreasing γ to −28.9° and increasing R and R CC to 2.797 and 3.814 Å, respectively. In the 1 8O substituted species, the CO2’s are rotated in the C–C–C plane from C 3v symmetry by the pinwheel angle β=∼32.5°.
92(1990); http://dx.doi.org/10.1063/1.458119View Description Hide Description
The photoelectron spectrum for the mixed singlet–triplet levels populated through excitation of the origin of the S 1 1 B 3u state in the intermediate case molecule pyrazine contains structure identifiable with the different spin components of the molecular eigenstates. The triplet part of the spectrum has a broad background and a few resolved peaks. It is suggested that the structured component is comprised of the transitions of a few high frequency vibrational combinations in the triplet, while the broad distribution of electron energies results from ionization of a dense manifold of low frequency combinations which are likely to undergo IVR. We also report the photoelectron spectrum of ‘‘relaxed’’ triplet states produced by collisional relaxation from levels populated after ISC from the origin of the S 1 state.
The origin of the superposition principle for circular intensity differential scattering by hierarchical chiral structures92(1990); http://dx.doi.org/10.1063/1.458120View Description Hide Description
The superposition principle states that the angle‐dependent difference in scattering intensities for right and left circularly polarized light, for chiral structures composed of various hierarchical levels, is the sum of the contributions from each level taken separately. Analytic expressions describing this superposition are obtained for oriented and rotationally averaged hierarchical chiral structures. It is shown that the superposition principle holds only when the coiling levels in the chiral structure are disparate. In particular, for a structure composed of two chiral levels, the lower order structure must be much smaller than both the wavelength of light and the higher order structure. Numerical calculations using these expressions are carried out for superhelices of varying dimensions.
92(1990); http://dx.doi.org/10.1063/1.458121View Description Hide Description
The x‐ray diffraction and the depolarization ratio of the ν1 Raman line were measured in liquid carbon tetrachloride. The previously proposed liquid structure models for an x‐ray diffraction pattern were found to be difficult to distinguish from each other. It has been reported that the relation between the local structure and its fluctuation corresponds to the depolarization ratio of the ν1 Raman line in liquid carbon tetrachloride [S. Hyodo and T. Fujiyama, Bull. Chem. Soc. Jpn. 5 3, 2456 (1980)]. The information obtained from this correspondence suggests that those models were clearly distinguishable. The contribution of the fluctuations of the molecular center and molecular orientation and that of the distortion from the temporarily assumed ideal lattice structure were also discussed to both the x‐ray diffraction curve and the depolarization ratio.
92(1990); http://dx.doi.org/10.1063/1.458122View Description Hide Description
The effect of pressure on the conformation of p‐terphenyl in carbon disulfide has been measured by using high pressureinfrared spectroscopy with a diamond anvil cell up to 7.1 kbar. Pressure effects on the nonplanar vibrational modes indicate that p‐terphenyl molecule in the solution gets more planar when compressed. The intensities at atmospheric pressure indicate that the conformation in the solution is predominantly in D 2 symmetry. Assuming that the two dihedral angles between the adjacent phenyl rings are equal to each other at all pressures, we estimate the rate of decrease of the dihedral angles by compression to be 2° kbar−1, which is equal to that of biphenyl in previous work.
92(1990); http://dx.doi.org/10.1063/1.458123View Description Hide Description
The microwave spectra of (H2CO)2 and (D2CO)2 have been observed with a pulsed beam, Fabry–Perot cavity, Fourier transformmicrowave spectrometer. Both species exhibit a‐type spectra which are split by internal rotation of each monomer unit and an interchange of donor–acceptor bonding roles analogous to the water dimer. Rotational analysis of each spectrum provides the constants A=18583.(54) MHz, 1/2 (B+C)=3272.105(34) MHz, and B−C=503.92(17) MHz for (H2 CO)2 and A=14 862.1(35) MHz, 1/2 (B+C)=3030.2366(37) MHz, and B−C=490.977(18) MHz for (D2CO)2. Stark effect measurements result in derived electric dipole components μ a =0.858(4) D and μ b =0.027(10) D for (H2 CO)2 and μ a =0.908(4) D and μ b =0.095(4) D for (D2CO)2. The geometry obtained from fitting the derived moments of inertia has the planes of the two monomer units perpendicular in a nearly antiparallel orientation of the CO groups with a center‐of‐mass distance of 3.046(17) Å. The shortest carbon to oxygen distance (2.98 Å) and hydrogen to oxygen distance (2.18 Å) between the monomer units are indicative of a dual bond interaction to form a ring structure.
92(1990); http://dx.doi.org/10.1063/1.458124View Description Hide Description
Vibrationally resolved HeI (584 Å) photoelectron spectra of the heavy group IV–VI diatomics SnSe, SnTe, PbSe, and PbTe were obtained with a new high temperature molecular beamsource.Ionization potentials and spectroscopic constants are reported for all the ionic states observed. Relativistic complete active space multiconfiguration self‐consistent field (MCSCF) followed by multireference singles+doubles relativistic configuration interaction (CI) calculations which included up to 200 000 configurations were made on both the neutral diatomics and their positive ions. Ionization potentials and spectroscopic constants were calculated and were in good agreement with the experimentally measured values. Relativistic CI potential energy curves were calculated for all the neutral ground states and the ionic states involved. Relativistic effects were shown to play an important role in these heavy diatomics. The 2Σ+ 1/2 and 2Π1 / 2 states for all four molecular ions showed avoided curve crossings, which resulted in pronounced shoulders in the Ω=1/2 potential energy curves of PbTe+. Experimentally, autoionization transitions were also observed for the PbTe+ spectrum. The importance of the relativistic effect and chemical bonding in the heavy diatomics are discussed.
92(1990); http://dx.doi.org/10.1063/1.458125View Description Hide Description
Laser induced fluorescencespectra of the ArOH bands associated with electronic excitation of the OH A–X(v’=0−v‘=0) transition were reported previously. An extensive search in the vicinity of the OH/OD A (2 Σ+ )−X(2 Π) system (v’=0−v‘=0 and v’=1−v‘=0) led to the observation of 36 vibronic bands which were identified as belonging to ArOH or ArOD complexes. All of the bands were found to originate from the 2 Π(3/2), v‘=0 ground electronic state of the OH/OD radical. Two distinguishable vibronic structures were identified. A lower energy vibrational progression was assigned to the van der Waals stretch. This band system was designated as the ‘‘A’’ bands. A higher energy set of bands exhibited a different vibrational structure which did not fit a simple vibrational mode progression. These bands were labeled as the ‘‘U’’ system. Spectra showing partially resolved rotational structure were recorded for all of the observed ArOH/ArOD bands. The A and U systems were also distinguished by differences in their rotational structure and effects of H/D substitution on the vibrational and rotational parameters. Preliminary rotational analysis gave ground state constants (B ‘ 0 ) of 0.100 cm− 1 for ArOH and 0.098 cm− 1 for ArOD. (B e constants for the excited state of the A system were determined to be 0.178 cm− 1 for ArOH and 0.168 cm− 1 for ArOD. These rotational constants show that the Ar–OH/Ar–OD van der Waals bond has been considerably shortened upon electronic excitation. This suggests that partial charge transfer might contribute to the stability of ArOH and ArOD complexes in the excited electronic state. Observed spectra and results of rovibronic analysis are discussed.
Elastic constants of the metastable orientationally disordered phase Ia of CCl4 by Brillouin spectroscopy92(1990); http://dx.doi.org/10.1063/1.458073View Description Hide Description
The technique of Brillouin spectroscopy has been used to determine the adiabatic elastic constants, including their temperature dependence, and the elasto‐optic coefficient ratios of single crystals of the metastable orientationally disordered phase Ia of CCl4. The values of the elastic constants at 244.1 K are C 1 1=35.7, C 1 2=31.6, and C 4 4=11.1 (in units of kbar) and the elasto‐optic coefficient ratios are P 1 2/P 1 1=0.85 and P 4 4/P 1 1=0.22. The elastic constants results suggest that there is strong rotation–translation coupling in the k=0 acoustic wave vector regime and that this coupling is likely responsible for the metastability of the fcc phase Ia with respect to the rhombohedral phase Ib.
92(1990); http://dx.doi.org/10.1063/1.458074View Description Hide Description
Detailed measurements of the complex dielectric function ε=ε’−iε‘ as functions of frequency between 45 MHz and 20 GHz and concentration c are presented and analyzed for LiCl/H2O solutions. In this frequency range, the dielectric response is due to free water molecules and the response of the bound water occurs at lower frequencies and is not observed. From the data, several parameters—the static and high frequency limiting values ε0 and ε∞, the dc conductivity σdc, the dielectric relaxation time τ d , and the hydration number n hyd—are extracted. The dissociation is observed to be incomplete suggesting the presence of undissociated ion pairs. A dissociation parameter is inferred from the σdc data. A model which views the liquid as composed of free water, hydrated single ions, and hydrated ion pairs is used to analyze the ε0 data. With inclusion of the dissociation parameter, the model describes the data reasonably well for c≤5.1 M. At higher concentrations, a unique relaxation time is not observed indicating glasslike behavior.
92(1990); http://dx.doi.org/10.1063/1.458075View Description Hide Description
We have conducted optically detected magnetic resonance(ODMR) experiments at pressure up to 40 kbar for neat biactyl (BA), neat benzil (BZ), and acetophenone (AP) doped in dibromobenzene (DBB). The pressure dependences of their zero‐field splitting (ZFS) parameters D and E are reported. For BA and BZ systems, the ‖D‖ value decreases greatly with increasing pressure. This behavior is in contrast with that of benzophenone (BP), whose ‖D‖ value increases sigmoidally 13% over the same pressure range. These results may be rationalized in a qualitative theory based on pressure modulation of the spin‐orbit coupling (SOC) contribution to the ZFS. ln aromatic ketones, lattice compression modifies the twist angle of the phenyl ring(s) relative to the carbonyl frame, thus changing the energy of the 3ππ* state relative to that of the 3nπ* state. This variation of the energy denominator in a second order perturbation enhances the SOC contribution to the ZFS. In comparison, the increase of spin–spin (SS) dipolar interaction by isotropic compression is relatively unimportant. Consistent with this picture, the very small 3ππ*–3nπ* energy gap produces an enormous pressure sensitivity of D and E in AP/DBB. The behavior of the ZFS in this case may be interpreted as a consequence of pressure tuning of the 3ππ* state through an anticrossing region. In addition, a new set of high frequency ODMR signals appears under pressure. This is attributed to a new site of AP having the 3nπ* as the phosphorescent triplet state. The pressure dependence of ZFS for benzil shows complicated fine structure. This is a testimony to the flexible nature of benzil in both the dihedral angle of the dicarbonyl fragment and the phenyl twist angle.
Mode‐specific infrared photodissociation of nitric oxide dimers: High‐resolution infrared spectroscopy of (14NO)2 and (15NO)292(1990); http://dx.doi.org/10.1063/1.458076View Description Hide Description
The high‐resolution infrared absorption spectra of the symmetric (ν1) and the antisymmetric NO stretching (ν4) bands of nitric oxide dimer (NO)2 have been measured for 14NO and 15NO in supersonic free jets. The ν1 and ν4 bands exhibit a dramatic difference in linewidth: approximately 200 MHz [full width at half‐maximum (FWHM)] for the ν1 band and approximately 5 GHz (FWHM) for the ν4 band. The predissociation lifetimes deduced from the linewidths are in excellent agreement with those reported in the recent time‐resolved measurement for 14NO [Casassa e t a l., J. Chem. Phys. 8 9, 1966 (1988)]. There is no systematic dependence of the linewidth on the rotational states of (NO)2. Isotope substitution does not influence the linewidths significantly. However, the ν4band structure of (15NO)2 is very different from that of (14NO)2, a difference that may be explained by a perturbation from a low‐lying singlet vibronic state. All of the experimental results obtained to date may be accounted for if it is assumed that the predissociation of (NO)2 is enhanced by an electronically nonadiabatic transition to a repulsive triplet surface. Vibrational potential coupling between the NO stretching and intermolecular modes, particularly an in‐plane NO bending mode, appears to play a key role in the mode specificity.
Infrared absorption spectroscopy of CO2–HX complexes using the CO2 asymmetric stretch chromophore: CO2HF(DF) and CO2HCl(DCl) linear and CO2HBr bent equilibrium geometries92(1990); http://dx.doi.org/10.1063/1.458077View Description Hide Description
Infrared absorption spectra associated with the CO2 asymmetric stretch vibration have been recorded for weakly bonded gas‐phase complexes of CO2 with HF, DF, HCl, DCl, and HBr, using tunable diode laser spectroscopy and a pulsed slit expansion (0.15×38 mm2) that provides >20 MHz overall resolution. Results obtained with CO2–HF are in agreement with earlier studies, in which the HF‐stretch region near 3900 cm− 1 was examined. In both cases, broad linewidths suggest subnanosecond predissociation. With CO2–DF, the natural linewidths are markedly narrower than with CO2–HF (e.g., 28 vs 182 MHz), and this difference is attributed to slower predissociation, possibly implicating resonances in the case of CO2–HF. Both CO2–HF and CO2–DF exhibited overlapping features: simple P and R branches associated with a linear rotor, and P and R branches containing doublets. As in earlier studies, the second feature can be assigned to either a slightly asymmetric rotor with K a =1, or a hot band involving a low‐frequency intermolecular bend mode.
Results obtained with CO2–HCl are in excellent agreement with earlier microwave measurements on the ground vibrational state, and the vibrationally excited state is almost identical to the lower state. Like CO2–DF, linewidths of CO2–HCl and CO2–DCl are much sharper than those of CO2–HF, and in addition, CO2–HCl and CO2–DCl exhibited weak hot bands, as were also evident with CO2–HF and CO2–DF. Upon forming complexes with either HF or HCl, the asymmetric stretch mode of CO2 underwent a blue shift relative to uncomplexed CO2. This can be understood in terms of the nature of the hydrogen bonds, and a b i n i t i o calculations are surprisingly good at predicting these shifts. Deuteration of both HF and HCl resulted in further blue shifts of the band origins. These additional shifts are attributed to stronger intermolecular interactions, i.e., deuteration lowers the zero‐point energy, and in a highly anharmonic field this results in a more compact average structure. While both HF and HCl complexes exhibit nearly linear geometries,CO2–HBr is asymmetric, with the Br–C symmetry line essentially perpendicular to the CO2 axis, and the H atom probably localized near one of the oxygens. Although the moments of inertia are insensitive to the location of the H atom in CO2–HBr, Bose–Einstein statistics require that odd K ‘ a states are missing for C 2v symmetry, as is observed with T‐shaped CO2–(rare gas) complexes. However, we observe a full complement of odd and even K a states, indicating that the H atom is not located symmetrically about the C 2v axis on the time scale of the measurement. With CO2–HBr, the low gas‐phase acidity of HBr and the high Br‐atom polarizability encourage a qualitative change in the geometry relative to CO2–HCl and CO2–HF. This has valuable implications for photoinitiated reactions in such complexes.
92(1990); http://dx.doi.org/10.1063/1.458078View Description Hide Description
The fluorescence excitation spectrum, the MPI spectrum, and the absorptionspectrum of acetylene were observed for the Ã 1 A u ←X̃ 1∑+ g band system in a static gas cell and in a supersonic jet. A sudden and drastic decrease in fluorescence quantum yield, Φ f , was found between the V 4 K 2 (46 339 cm−1 ) and 21 V 3 K 0 (46 673 cm−1 ) sublevels. The decrease in Φ f is concluded to be due to the predissociation into C2 H+H. J dependence of Φ f was found for the V 4 K 1 vibronic sublevel, while it is absent for the level lying above V 4 K 1 . From the observed J dependence, the predissociation mechanism was concluded to be vibrational predissociation by tunneling through a potential barrier.
Differential anisotropy of polarizability measured by picosecond transient dichroism and birefringence92(1990); http://dx.doi.org/10.1063/1.458079View Description Hide Description
The difference in the anisotropy of the polarizability between ground and excited states for two dyes in ethanol was first directly measured by Waldeck e t a l. using transient molecular dichroism and the birefringence method. In this paper, we present the development of the theoretical description of this method and we measure the differential polarizability for the dyes DODCI, sulforhodamine 101, and nile blue in various alcohols. The following values were obtained: <+200 Å3, +590 Å3, and +720 Å3, respectively, independent of the solvent. The proposed theory was used in a computer simulation and good agreement of the observed and calculated values was obtained. In the experiment, the rotational relaxation times were also determined and compared to the results obtained for the same molecules from the measurements using the up‐conversion technique. The difference in measured rotational times for dyes in more viscous alcohols using both experimental techniques suggests that the rotational diffusion is different in the ground and excited states.
A theoretical treatment of multiple quantum nuclear spin coherences in electron spin echo studies of polyacetylene92(1990); http://dx.doi.org/10.1063/1.458080View Description Hide Description
We present a theoretical investigation of the origin of the unusual 3‐pulse electron spin echo envelope modulation (ESEEM) pattern obtained from 1 3C enriched polyacetylene, which exhibits modulation at multiple harmonics of the nuclear Larmor frequency ω n . ESEEM signals have been calculated for different models of the electron–nuclear spin coupling. It is shown that the local hyperfine tensors observed in electron nuclear double resonance(ENDOR) experiments on (1 3CH) x cannot account for the ESEEM results. These tensors have isotropic components which are too large to give narrow peaks at ω n , and there are too few nuclei to produce the large number of harmonics observed. Simulations for a point electron coupled to nuclei on a spherical shell, the spherical model, give insight into the conditions required to obtain narrow lines at multiple harmonics of ω n : The electron must be weakly coupled to a very large number of nuclei. Analytical results show that for the spherical model in the limit of small coupling, the intensity profile of the Fourier transform of the three‐pulse ESEEM will be a Gaussian of width k̄ N, where k̄ is a function of the magnitude of the electron–nuclear coupling and N is the number of nuclei. Simulations assuming that the unpaired electron is delocalized in one dimension along the polyene chain and dipolar coupled to the 1 3C nuclei on neighboring chains give multiple harmonic patterns, at a reduced interchain distance. We conclude that the observed ESEEM pattern arises from nuclei on neighboring polyacetylene chains and is closely related to the matrix/distant ENDOR line observed for (1 3CH) x .
92(1990); http://dx.doi.org/10.1063/1.458081View Description Hide Description
The quantum yields of the products, OH(X 2Π), O(3 P) [plus O(1 D)] and H(2 S), in the photolysis of H2O2 and CH3OOH at 248 nm and 298 K have been measured. OH was directly observed by laser‐induced fluorescence while the atomic species were detected by cw‐resonance fluorescence. All quantum yield measurements were made using relative methods. The quantum yields of OH, O, and H in H2O2photolysis were measured relative to the well known quantum yields of O(1 D) and O(3 P) in the photodissociation of O3, and H(2 S) in CH3SH. The values we obtain are, 2.09±0.36, <0.002 and <0.0002 for OH, O, and H, respectively. For CH3OOH photolysis, the quantum yield of OH was measured relative to our value for OH quantum yield in H2O2photolysis, and the quantum yields of O and H relative to those in O3 and CH3SH photodissociation, respectively. The values we obtain are, 1.00±0.18, <0.007 and 0.038±0.007 for OH, O, and H, respectively. In both H2O2 and CH3OOH photolysis, the observed O and H quantum yields showed an apparent dependence on the fluence of the photolysis light, the possible origin of which is discussed. The large quantum yield of OH we measure is consistent with the known continuous and unstructured absorption spectra of these molecules in this wavelength region, where the most important process is the dissociative (Ã 1 A←X̃ 1 A) transition to give OH(X 2Π, v‘=0) fragment.