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1.
J Chem Phys ; 152(4): 041105, 2020 Jan 31.
Artigo em Inglês | MEDLINE | ID: mdl-32007030

RESUMO

The C2 carbon cluster is found in a large variety of environments including flames, electric discharges, and astrophysical media. Due to spin-selection rules, assessing a complete overview of the dense vibronic landscape of the C2 + cation starting from the ground electronic state X Σg+1 of the neutral is not possible, especially since the C2 + ground state is of X+ Σg-4 symmetry. In this work, a flow-tube reactor source is employed to generate the neutral C2 in a mixture of both the lowest singlet X Σg+1 and triplet a 3Πu electronic states. We have investigated the vibronic transitions in the vicinity of the first adiabatic ionization potential via one-photon ionization with vacuum ultraviolet synchrotron radiation coupled with electron/ion double imaging techniques. Using ab initio calculations and Franck-Condon simulations, three electronic transitions are identified and their adiabatic ionization energy is determined Ei(a+ 2Πu←X 1Σg +)=12.440(10) eV, Ei(X+ 4Σg -←a 3Πu)=11.795(10) eV, and Ei(a+2Πu ← a3Πu) = 12.361(10) eV. From the three origin bands, the following energy differences are extracted: ΔE(a - X) = 0.079(10) eV and ΔE(a+ - X+) = 0.567(10) eV. The adiabatic ionization potential corresponding to the forbidden one-photon transition X+ ← X is derived and amounts to 11.873(10) eV, in very good agreement with the most recent measurement by Krechkivska et al. [J. Chem. Phys. 144, 144305 (2016)]. The enthalpy of formation of the doublet ground state C2 + cation in the gas phase is determined at 0 K, ΔfH0(0K)(C2 +(Πu2))=2019.9(10) kJ mol-1. In addition, we report the first experimental ion yield of C2 for which only a simple estimate was used up to now in the photochemistry models of astrophysical media due to the lack of experimental data.

2.
Phys Chem Chem Phys ; 21(46): 25907-25915, 2019 Nov 27.
Artigo em Inglês | MEDLINE | ID: mdl-31742264

RESUMO

We present the absolute photoionization cross-section of the mercapto radical, SH, recorded from its first ionization energy at 10.4 eV up to a photon energy of 15 eV. The absolute scale was calibrated at the fixed photon energy of 11.2 eV using the known values of H2S and S as references. SH and S were produced in a microwave discharge flow-tube reactor by hydrogen abstraction of the H2S precursor. The measured photoionization cross-section of SH dramatically differs from the one currently employed to model the presence of this species in a number of astronomical environments, where SH along with its ionic counterpart SH+ have been detected. The cation spectroscopy and fragmentation of H2S, SH and S in the 9.2-15.0 eV energy range obtained using threshold photoelectron techniques is also presented and discussed in the context of existing literature.

3.
J Phys Chem A ; 123(42): 9193-9198, 2019 Oct 24.
Artigo em Inglês | MEDLINE | ID: mdl-31556614

RESUMO

We report on the photoionization of the resonance-stabilized anilino radical (C6H5NH) formed by H atom abstraction from aniline by F atoms in a flow tube. The spectra were recorded from 7.8 to 9.7 eV by using a double-imaging photoelectron/photoion coincidence spectrometer with VUV radiation provided by the DESIRS beamline at the SOLEIL synchrotron. The vibrationally resolved recorded threshold photoelectron spectrum of the anilino radical showed transitions to the ground X+1A' ← X2A″ and first excited states a+3A″ ← X2A″ of the cation, which were assigned through comparison with theoretically simulated spectra, yielding an adiabatic ionization energy of 8.02 ± 0.02 eV. These results are discussed in light of existing data on the picolyl structural isomers and are of interest for the analytical applications of coincidence techniques in real-time combustion analysis where these intermediates are found.

4.
J Chem Phys ; 150(24): 244303, 2019 Jun 28.
Artigo em Inglês | MEDLINE | ID: mdl-31255068

RESUMO

The results of an extensive ab initio study of the cyanobutadiyne cation, initially motivated by threshold-photoelectron spectroscopy experiments [see the study by Gans et al., J. Chem. Phys. 150, 244304 (2019)], are reported in the present paper. Calculations at the internally contracted multireference configuration interaction level of theory have been performed to derive the rovibronic properties of the seven lowest electronic states of HC5N+. Equilibrium geometries, rotational constants, vibrational frequencies, electric dipole moments, and spin-orbit constants have been calculated and compared with experimental data when available. Adiabatic and vertical ionization energies from the neutral ground state as well as transition energies within the cation electronic manifold are predicted, using the convergence to the complete basis set limit. The accurate description of the complex energy landscape up to 32 000 cm-1 above the ionization potential allows us to perform Franck-Condon simulations of the photoionization spectrum to the X+ 2Π, A+ 2Π, B+ 2Σ+, and C+ 2Π states and allows us to simulate the A+ 2Π â†’ X+ 2Π emission spectrum. The vibronic perturbations occurring on the excited potential energy surfaces are revealed and discussed, in particular, for the 3 2Π surface, which presents a double-well topography.

5.
J Chem Phys ; 150(24): 244304, 2019 Jun 28.
Artigo em Inglês | MEDLINE | ID: mdl-31255091

RESUMO

We report the vacuum-ultraviolet threshold-photoelectron spectrum of HC5N recorded over a wide spectral range, from 84 000 to 120 000 cm-1, with a 120 cm-1 spectral resolution, better than what was achieved in previous photoelectron studies, and with mass selectivity. The adiabatic ionization potential of cyanobutadiyne is measured at 85 366 (±40) cm-1. Assignment of the vibrational bands of the four lowest electronic states X+2Π, A+2Π, B+2Σ+, and C+2Π are performed, supported by high level ab initio calculations which are fully detailed in Paper II [B. Gans et al., J. Chem. Phys. 150, 244303 (2019)] and by Franck-Condon simulations. Only vibrational stretching modes are observed in the threshold-photoelectron spectra. The ground state of HC5N+ exhibits a vibrational progression in the ν2 stretching mode involving mainly the elongation of the C≡C triple bonds, whereas the A+ and C+ excited electronic states show a progression in the stretching mode mainly associated with the elongation of the C≡N bond, i.e., ν4 and ν3, respectively. The B+ state appears almost as a vibrationless structure in close vicinity to the A+ state.

6.
Phys Chem Chem Phys ; 21(5): 2337-2344, 2019 Jan 30.
Artigo em Inglês | MEDLINE | ID: mdl-30656348

RESUMO

The photoelectron spectrum of the X1Σ+ → X+2Σ+ ionizing transition of hydrogen isocyanide (HNC) is measured for the first time at a fixed photon energy (13 eV). The assignment of the spectrum is supported by wave-packet calculations simulating the photoionization transition spectrum and using ab initio calculations of the potential energy surfaces for the three lowest electronic states of the cation. The photoelectron spectrum allows the retrieval of the fundamental of the CN stretching mode of the cationic ground state ([small nu, Greek, tilde]3 = 2260 ± 80 cm-1) and the adiabatic ionization energy of hydrogen isocyanide: IE(HNC) = 12.011 ± 0.010 eV, which is far below that of HCN (IE(HCN) = 13.607 eV). In light of this latter result, the thermodynamics of the HCN+/HNC+ isomers is discussed and a short summary of the values available in the literature is given.

7.
Phys Chem Chem Phys ; 20(13): 8707-8718, 2018 Mar 28.
Artigo em Inglês | MEDLINE | ID: mdl-29537039

RESUMO

We present the photoelectron spectra of C3Hx (x = 0-3) formed in a microwave discharge flow-tube reactor by consecutive H abstractions from C3H4 (C3Hx + F → C3Hx-1 + HF (x = 1-4)), but also from F + CH4 schemes by secondary reactions. The spectra were obtained combining tunable VUV synchrotron radiation with double imaging electron/ion coincidence techniques, yielding mass-selected threshold photoelectron spectra. The obtained results complement not only existing ones, but for the first time the photoelectron spectra of C3, cyclic and linear C3H (c,l-C3H) as well as of the excited states of C3H3 are reported. In the case of c-C3H, l,t-C3H2 and C3H3, Franck-Condon simulations have been performed in order to assign the vibrational structure. The adiabatic ionization energies of these radicals are reported and compared to ab initio calculated values as well as to theoretical values using known enthalpies of formation.

8.
J Chem Phys ; 148(5): 054302, 2018 Feb 07.
Artigo em Inglês | MEDLINE | ID: mdl-29421906

RESUMO

The line intensity of photoelectron spectra when either the neutral or cationic species display a Renner-Teller coupling is derived and applied to the modeling of the photoelectron spectra of CNC, CCN, and HCCN. The rovibronic energy levels of these three radicals and of their cations are investigated starting from ab initio results. A model treating simultaneously the bending mode and the overall rotation is developed to deal with the quasilinearity problem in CNC+, CCN+, and HCCN and accounts for the large amplitude nature of their bending mode. This model is extended to treat the Renner-Teller coupling in CNC, CCN, and HCCN+. Based on the derived photoelectron line intensity, the photoelectron spectra of all three molecules are calculated and compared to the experimental ones.

9.
J Phys Chem Lett ; 8(17): 4038-4042, 2017 Sep 07.
Artigo em Inglês | MEDLINE | ID: mdl-28796511

RESUMO

The cyano radical is a ubiquitous molecule and was, for instance, one of the first species detected in astrophysical media such as comets or diffuse clouds. In photodissociation regions, the reaction rate of CN+ + CO → CN + CO+ is one of the critical parameters defining nitrile chemistry. The enthalpy of this charge transfer reaction is defined as the difference of ionization energies (EI) between CN and CO. Although EI(CO) is known accurately, the EI(CN) values are more dispersed and deduced indirectly from thermodynamic thresholds only, all above EI(CO), leading to the assumption that the reaction is fast even at low temperature. Using a combination of synchrotron radiation, electron/ion imaging coincidence techniques, and supporting ab initio calculations, we directly determine the first adiabatic ionization energy of CN at 13.956(7) eV, and we demonstrate that EI(CN) < EI(CO). The findings suggest a very slow reaction in the cold regions of interstellar media.

10.
J Chem Phys ; 147(1): 013908, 2017 Jul 07.
Artigo em Inglês | MEDLINE | ID: mdl-28688448

RESUMO

We present the photoelectron spectroscopy of four radical species, CHxCN (x = 0-2) and CNC, formed in a microwave discharge flow-tube reactor by consecutive H abstractions from CH3CN (CHxCN + F → CHx-1CN + HF (x = 1-3)). The spectra were obtained combining tunable vacuum ultraviolet synchrotron radiation with double imaging electron/ion coincidence techniques, which yielded mass-selected threshold photoelectron spectra. The results obtained for H2CCN complement existing ones while for the other radicals the data represent the first observation of their (single-photon) ionizing transitions. In the case of H2CCN, Franck-Condon calculations have been performed in order to assign the vibrational structure of the X+ 1A1←X 2B1 ionizing transition. A similar treatment for the HCCN, CCN, and CNC radicals appeared to be more complicated mainly because a Renner-Teller effect strongly affects the vibrational levels of the ground electronic state of the HCCN+, CCN, and CNC species. Nevertheless, the first adiabatic ionization energies of these radicals are reported and compared to our ab initio calculated values, leading to new values for enthalpies of formation (ΔfH2980(HCCN+(X2A'))=1517±12kJmol-1,ΔfH2980(CCN(X2Π))=682±13kJmol-1, and ΔfH2980(CNC(X2Πg))=676±12kJmol-1), which are of fundamental importance for astrochemistry.

11.
J Chem Phys ; 145(23): 234309, 2016 Dec 21.
Artigo em Inglês | MEDLINE | ID: mdl-28010104

RESUMO

Vacuum-ultraviolet pulsed-field-ionization zero-kinetic-energy photoelectron spectra of X+Π2←XΣ+1 and B+Π2←XΣ+1 transitions of the HC314N and HC315N isotopologues of cyanoacetylene have been recorded. The resolution of the photoelectron spectra allowed us to resolve the vibrational structures and the spin-orbit splittings in the cation. Accurate values of the adiabatic ionization potentials of the two isotopologues (EI/hc(HC314N)=93 909(2) cm-1 and EI/hc(HC315N)=93 912(2) cm-1), the vibrational frequencies of the ν2, ν6, and ν7 vibrational modes, and the spin-orbit coupling constant (ASO = -44(2) cm-1) of the X+Π2 cationic ground state have been derived from the measurements. Using ab initio calculations, the unexpected structure of the B+Π2←XΣ+1 transition is tentatively attributed to a conical intersection between the A+ and B+ electronic states of the cation.

12.
J Chem Phys ; 145(23): 234310, 2016 Dec 21.
Artigo em Inglês | MEDLINE | ID: mdl-28010106

RESUMO

Threshold-photoionization spectroscopy of cyanoacetylene (HC3N) and its 15N isotopologue has been investigated in the vacuum-ultraviolet range with a synchrotron-based experiment allowing to record threshold-photoelectron spectrum and photoion yield over a large energy range (from 88 500 to 177 500 cm-1, i.e., from 11 to 22 eV). Adiabatic ionization energies towards the three lowest electronic states X+2Π, A+ Σ+2, and B+ Π2 are derived from the threshold-photoelectron spectrum. A detailed description of the vibrational structure of these states is proposed leading to the determination of the vibrational frequencies for most modes. The vibrational assignments and the discussion about the electronic structure are supported by multireference ab initio calculations (CASPT2, MRCI). Unprecedented structures are resolved and tentatively assigned in the region of the B+← X transition. Exploratory calculations highlight the complexity of the electronic landscape of the cation up to approximately 10 eV above its ground state.

13.
J Phys Chem A ; 119(50): 12339-48, 2015 Dec 17.
Artigo em Inglês | MEDLINE | ID: mdl-26469080

RESUMO

The room-temperature photoabsorption spectra of a number of linear alkynes with internal triple bonds (e.g., 2-butyne, 2-pentyne, and 2- and 3-hexyne) show similar resonances just above the lowest ionization threshold of the neutral molecules. These features result in a substantial enhancement of the photoabsorption cross sections relative to the cross sections of alkynes with terminal triple bonds (e.g., propyne, 1-butyne, 1-pentyne, ...). Based on earlier work on 2-butyne [ Xu et al., J. Chem. Phys. 2012, 136, 154303 ], these features are assigned to excitation from the neutral highest occupied molecular orbital (HOMO) to a shape resonance with g (l = 4) character and approximate π symmetry. This generic behavior results from the similarity of the HOMOs in all internal alkynes, as well as the similarity of the corresponding gπ virtual orbital in the continuum. Theoretical calculations of the absorption spectrum above the ionization threshold for the 2- and 3-alkynes show the presence of a shape resonance when the coupling between the two degenerate or nearly degenerate π channels is included, with a dominant contribution from l = 4. These calculations thus confirm the qualitative arguments for the importance of the l = 4 continuum near threshold for internal alkynes, which should also apply to other linear internal alkynes and alkynyl radicals. The 1-alkynes do not have such high partial waves present in the shape resonance. The lower l partial waves in these systems are consistent with the broader features observed in the corresponding spectra.

14.
J Phys Chem A ; 117(39): 9353-62, 2013 Oct 03.
Artigo em Inglês | MEDLINE | ID: mdl-23231536

RESUMO

The X(+ 2)Π â† X (1)Σ(+) photoionizing transition of iodoacetylene (HC2I) has been investigated by pulsed-field-ionization zero-kinetic-energy (PFI-ZEKE) photoelectron spectroscopy. The resolution of the rotational structure of the spectra and its analysis provided information on the structure of the HC2I(+) cation and the photoionization dynamics of HC2I. In the ground electronic (2)Π state, the HC2I(+) cation is found to be linear and subject to a strong spin-orbit coupling. The first adiabatic ionization energy of HC2I and the spin-orbit splitting of the X(+ 2)Π ground state of HC2I(+) were determined to be EI(HC2I)/hc = 78296.5(2) cm(-1) and Δν̃SO = 3257(1) cm(-1), respectively. The large spin-orbit interaction almost entirely masks the Renner-Teller effect, which is only detectable through the observation of the nominally forbidden transition to the first excited level (5(1)) of the HCC-I bending mode ν5. The interaction of ∼2 cm(-1) observed between the 5(1) levels of (2)Σ1/2 and (2)Δ5/2 symmetry is attributed to a vibronic interaction with the B (2)Σ(+) electronic state of HC2I(+). The spin-orbit energy level structure of tri- and tetra-atomic molecules subject to the Renner-Teller effect and spin-orbit coupling is discussed for the two limiting cases where the spin-orbit-coupling constant is much smaller and much larger than the bending-mode frequencies.

15.
J Phys Chem A ; 115(21): 5387-96, 2011 Jun 02.
Artigo em Inglês | MEDLINE | ID: mdl-21548615

RESUMO

The absolute photoionization cross section of C(2)H(5) has been measured at 10.54 eV using vacuum ultraviolet (VUV) laser photoionization. The C(2)H(5) radical was produced in situ using the rapid C(2)H(6) + F → C(2)H(5) + HF reaction. Its absolute photoionization cross section has been determined in two different ways: first using the C(2)H(5) + NO(2) → C(2)H(5)O + NO reaction in a fast flow reactor, and the known absolute photoionization cross section of NO. In a second experiment, it has been measured relative to the known absolute photoionization cross section of CH(3) as a reference by using the CH(4) + F → CH(3) + HF and C(2)H(6) + F → C(2)H(5) + HF reactions successively. Both methods gave similar results, the second one being more precise and yielding the value: σ(C(2)H(5))(ion) = (5.6 ± 1.4) Mb at 10.54 eV. This value is used to calibrate on an absolute scale the photoionization curve of C(2)H(5) produced in a pyrolytic source from the C(2)H(5)NO(2) precursor, and ionized by the VUV beam of the DESIRS beamline at SOLEIL synchrotron facility. In this latter experiment, a recently developed ion imaging technique is used to discriminate the direct photoionization process from dissociative ionization contributions to the C(2)H(5)(+) signal. The imaging technique applied on the photoelectron signal also allows a slow photoelectron spectrum with a 40 meV resolution to be extracted, indicating that photoionization around the adiabatic ionization threshold involves a complex vibrational overlap between the neutral and cationic ground states, as was previously observed in the literature. Comparison with earlier photoionization studies, in particular with the photoionization yield recorded by Ruscic et al. is also discussed.

16.
Phys Chem Chem Phys ; 13(18): 8140-52, 2011 May 14.
Artigo em Inglês | MEDLINE | ID: mdl-21340077

RESUMO

Methane photolysis has been performed at the two Vacuum UltraViolet (VUV) wavelengths, 121.6 nm and 118.2 nm, via a spectrally pure laser pump-probe technique. The first photon is used to dissociate methane (either at 121.6 nm or at 118.2 nm) and the second one is used to ionise the CH(2) and CH(3) fragments. The radical products, CH(3)(X), CH(2)(X), CH(2)(a) and C((1)D), have been selectively probed by mass spectrometry. In order to quantify the fragment quantum yields from the mass spectra, the photoionisation cross sections have been carefully evaluated for the CH(2) and CH(3) radicals, in two steps: first, theoretical ab initio approaches have been used in order to determine the pure electronic photoionisation cross sections of CH(2)(X) and CH(2)(a), and have been rescaled with respect to the measured absolute photoionisation cross section of the CH(3)(X) radical. In a second step, in order to take into account the substantial vibrational energy deposited in the CH(3)(X) and CH(2)(a) radicals, the variation of their cross sections near threshold has been simulated by introducing the pertinent Franck-Condon overlaps between neutral and cation species. By adding the interpolated values of CH quantum yields measured by Rebbert and Ausloos [J. Photochem., 1972, 1, 171-176], a complete set of fragment quantum yields has been derived for the methane photodissociation at 121.6 nm, with carefully evaluated 1σ uncertainties: Φ[CH(3)(X)] = 0.42 ± 0.05, Φ[CH(2)(a)] = 0.48 ± 0.05, Φ[CH(2)(X)] = 0.03 ± 0.08, Φ[CH(X)] = 0.07 ± 0.01. These new data have been measured independently of the H atom fragment quantum yield, subject to many controversies in the literature. From our results, we evaluate Φ(H) = 0.55 ± 0.17 at 121.6 nm. The quantum yields for the photolysis at 118.2 nm differ notably from those measured at 121.6 nm, with a substantial production of the CH(2)(X) fragment: Φ[CH(3)(X)] = 0.26 ± 0.04, Φ[CH(2)(a)] = 0.17 ± 0.05, Φ[CH(2)(X)] = 0.48 ± 0.06, Φ[CH(X)] = 0.09 ± 0.01, Φ(H) = 1.31 ± 0.13. These new data should bring reliable and essential inputs for the photochemical models of the Titan atmosphere.

17.
J Phys Chem A ; 114(14): 4818-30, 2010 Apr 15.
Artigo em Inglês | MEDLINE | ID: mdl-20218643

RESUMO

Threshold photoelectron spectra (TPES) of the isotopomers of the methyl radical (CH(3), CH(2)D, CHD(2), and CD(3)) have been recorded in the 9.5-10.5 eV VUV photon energy range using third generation synchrotron radiation to investigate the vibrational spectroscopy of the corresponding cations at a 7-11 meV resolution. A threshold photoelectron-photoion coincidence (TPEPICO) spectrometer based on velocity map imaging and Wiley-McLaren time-of-flight has been used to simultaneously record the TPES of several radical species produced in a Ar-seeded beam by dc flash-pyrolysis of nitromethane (CH(x)D(y)NO(2), x + y = 3). Vibrational bands belonging to the symmetric stretching and out-of-plane bending modes have been observed and P, Q, and R branches have been identified in the analysis of the rotational profiles. Vibrational configuration interaction (VCI), in conjunction with near-equilibrium potential energy surfaces calculated by the explicitly correlated coupled cluster method CCSD(T*)-F12a, is used to calculate vibrational frequencies for the four radical isotopomers and the corresponding cations. Agreement with data from high-resolution IR spectroscopy is very good and a large number of predictions is made. In particular, the calculated wavenumbers for the out-of-plane bending vibrations, nu(2)(CH(3)(+)) = 1404 cm(-1), nu(4)(CH(2)D(+)) = 1308 cm(-1), nu(4)(CHD(2)(+)) = 1205 cm(-1), and nu(2)(CD(3)(+)) = 1090 cm(-1), should be accurate to ca. 2 cm(-1). Additionally, computed Franck-Condon factors are used to estimate the importance of autoionization relative to direct ionization. The chosen models globally account for the observed transitions, but in contrast to PES spectroscopy, evidence for rotational and vibrational autoionization is found. It is shown that state-selected methyl cations can be produced by TPEPICO spectroscopy for ion-molecule reaction studies, which are very important for the understanding of the planetary ionosphere chemistry.

18.
J Phys Chem A ; 114(9): 3237-46, 2010 Mar 11.
Artigo em Inglês | MEDLINE | ID: mdl-19947606

RESUMO

A pyrolysis source coupled to a supersonic expansion has been used to produce the CH3 radical from two precursors, iodomethane CH3I and nitromethane CH3NO2. The relative ionization yield of CH3 has been recorded at the SOLEIL Synchrotron Radiation source in the range 9.0-11.6 eV, and its ionization threshold has been modeled by taking into account the vibrational and rotational temperature of the radical in the molecular beam. The relative photoionization yield has been normalized to an absolute cross section scale at a fixed wavelength (118.2 nm, sigma(i)(CH3) = 6.7(-1.8)(+2.4) Mb, 95% confidence interval) in an independent laboratory experiment using the same pyrolysis source, a vacuum ultraviolet (VUV) laser, and a carefully calibrated detection chain. The resulting absolute cross section curve is in good agreement with the recently published measurements by Taatjes et al., although with an improved signal-to-noise ratio. The absolute photoionization cross section of CH3I at 118.2 nm has also been measured to be sigma(i)(CH3I) = (48.2 +/- 7.9) Mb, in good agreement with previous electron impact measurements. Finally, the photoionization yield of the iodine atom in its ground state 2P(3/2) has been recorded using the synchrotron source and calibrated for the first time on an absolute cross section scale from our fixed 118.2 nm laser measurement, sigma(i)(I2P(3/2)) = 74(-23)(+33) Mb (95% confidence interval). The ionization curve of atomic iodine is in good agreement, although with slight variations, with the earlier relative ionization yield measured by Berkowitz et al. and is also compared to an earlier calculation of the iodine cross section by Robicheaux and Greene. It is demonstrated that, in the range of pyrolysis temperature used in this work, all the ionization cross sections are temperature-independent. Systematic care has been taken to include all uncertainty sources contributing to the final confidence intervals for the reported results.

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