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201.
Ospina MP Powell DH Yost RA 《Journal of the American Society for Mass Spectrometry》2003,14(2):102-109
The efficiency of the collision-induced dissociation (CID) process as a function of the internal energy deposited into the ion during the ionization event was evaluated. (M + H)+ ions of pyrrole, pyrrolidine, pyridine and piperidine (five and six-membered ring heterocyclics) were generated by chemical ionization (CI). The internal energy of the ions was varied by using different reagent gases. Both high-energy (keV) and low-energy (eV) CID were performed on these ions. The experiments showed that the (M + H)+ ions of the five-membered ring compounds, pyrrole and pyrrolidine, have higher fragmentation efficiencies than the six-membered ring compounds, pyridine and piperidine. Fragmentation efficiencies in high-energy CID clearly correlate with the internal energy deposited by the ionization technique. Experiments showed that the low-energy CID process is more sensitive than high-energy CID to changes in internal energy. 相似文献
202.
We present tandem quadrupole mass spectrometric data for 26 organophosphate and 16 carbamate pesticides. These data are of use in developing methods for screening samples for specific compounds (by selected reaction monitoring) or specific classes of compounds (by parent scans and neutral loss scans). Optimization of tandem mass spectrometry conditions for maximum sensitivity is also described. 相似文献
203.
Robert Grone Charles R. Johnson SA Eduardo Henry Wolkowicz 《Linear and Multilinear Algebra》2013,61(4):389-393
As a step toward understanding the unsolved problem of determining how large the permanent of a positive semi-definite matrix can be, given the eigenvalues, we note that a necessary condition for A to be a permanent maximizing matrix is that A commute with its permanental adjoint. 相似文献
204.
Modulation-filterbank models discard phase information above very low rates of amplitude modulation (AM). The present work evaluated this restriction by measuring thresholds for discriminating the starting phase of sinusoidal modulators of wideband-noise carriers. Results showed a low-pass characteristic with some listeners unable to perform the task once the modulation rate was greater than 12.5 Hz. For others, however, thresholds were obtained with AM rates of up to one to two octaves higher. Intersubject variability may in part relate to the presence of multiple discrimination cues, with only some based on comparison of the ongoing pattern of envelope fluctuation. 相似文献
205.
Matthew J. Pollard Christopher K. Hilton Hongli Li Kimberly Kaplan Richard A. Yost Herbert H. HillJr. 《International Journal for Ion Mobility Spectrometry》2011,14(1):15-22
Since the development of electrospray ionization (ESI) for ion mobility spectrometry mass spectrometry (IMMS), IMMS have been
extensively applied for characterization of gas-phase bio-molecules. Conventional ion mobility spectrometry (IMS), defined
as drift tube IMS (DT-IMS), is typically a stacked ring design that utilizes a low electric field gradient. Field asymmetric
ion mobility spectrometry (FAIMS) is a newer version of IMS, however, the geometry of the system is significantly different
than DT-IMS and data are collected using a much higher electric field. Here we report construction of a novel ambient pressure
dual gate DT-IMS coupled with a FAIMS system and then coupled to a quadrupole ion trap mass spectrometer (QITMS) to form a
hybrid three-dimensional separation instrument, DT-IMS-FAIMS-QITMS. The DT-IMS was operated at ~3 Townsend (electric field/number
density (E/N) or (Td)) and was coupled in series with a FAIMS, operated at ~80 Td. Ions were mobility-selected by the dual
gate DT-IMS into the FAIMS and from the FAIMS the ions were detected by the QITMS for as either MS or MSn. The system was evaluated using cocaine as an analytical standard and tested for the application of separating three isomeric
tri-peptides: tyrosine-glycine-tryptophan (YGW), tryptophan-glycine-tyrosine (WGY) and tyrosine-tryptophan-glycine (YWG).
All three tri-peptides were separated in the DT-IMS dimension and each had one mobility peak. The samples were partially separated
in the FAIMS dimension but two conformation peaks were detected for the YWG sample while YGW and WGY produced only one peak.
Ion validation was achieved for all three samples using QITMS. 相似文献
206.
Listeners' ability to discriminate interaural time difference (ITD) changes in low-frequency noise was determined as a function of differences in the noise spectra delivered to each ear. An ITD was applied to Gaussian noise, which was bandpass filtered using identical high-pass, but different low-pass cutoff frequencies across ears. Thus, one frequency region was dichotic, and a higher-frequency region monotic. ITD thresholds increased as bandwidth to one ear (i.e., monotic bandwidth) increased, despite the fact that the region of interaural spectral overlap remained constant. Results suggest that listeners can process ITD differences when the spectra at two ears are moderately different. 相似文献
207.
208.
We present a quantum-chemical analysis of the relationship between the bond length alteration (BLA) and the static first hyperpolarizability of a series of one-dimensional (1D) chromophores with donor-bridge-acceptor (D-B-A) structures. The calculated results show that the parameter BLA can be considered as an indicator to evaluate the molecular first hyper- polarizability. Along the direction of molecular ground-state dipole moments, the evolutions of BLA can be classified into three categories: the first is a non-monotonic line, which represents most chromophores; the second is monotonic increasing; and the third, contrarily, is monotonic decreasing. On the whole, the first hyperpolarizabilities of these studied chromophores are the monotonic functions of BLA along the direction of dipole moments. Therefore, the first hyperpolarizability of these 1D chromophores can be preliminarily evaluated in terms of the development of BLA without a rigorous computation. In other words, one can roughly estimate the relative magnitude of the first hyperpolarizability according to the optimized geometry. 相似文献
209.
The interaction of Cd2+ with uracil,2-thiouracil,4-thiouracil and 2,4-dithiouracil have been investigated by the density functional theory(DFT) calculations.For uracil and 2,4-dithiouracil,where the two basic sites are the same,Cd2+ attachment to the heteroatom at position 4 is preferred.However,for the systems where both types of basic centers,a carbonyl and a thiocarbonyl groups,are present,Cd2+ association with sulfur is favorable.The enhanced stability of these enolic and thiol forms comes from Cd2+ interaction with two basic sites simultaneously,which thereby triggers a significant aromatization of the ring.More significantly,the Cd2+ binding energy with uracil and its thio-derivatives is larger than the tautomerization barriers connecting the diketo-like forms with the corresponding enolic-like tautomers.Consequently,when associated with Cd2+,all tautomers are energetically accessible and should be observed in the gas phase. 相似文献
210.
Dye RH Brown CA Gallegos JA Yost WA Stellmack MA 《The Journal of the Acoustical Society of America》2006,120(6):3946-3956
This study examined the deleterious effects of a later-arriving sound on the processing of interaural differences of time (IDTs) from a preceding sound. A correlational analysis assessed the relative weight given to IDTs of source and echo clicks for echo delays of 1-64 ms when the echo click was attenuated relative to the source click (0-36 dB). Also measured were proportion correct and the proportion of responses predicted from the weights. The IDTs of source and echo clicks were selected independently from Gaussian distributions (mu=0 s, sigma = 100/s). Listeners were instructed to indicate the laterality of the source click. Equal weight was given to the source and echo clicks for echo delays of 64 ms with no echo attenuation. For echo delays of 16-64 ms, attenuating the echo had no substantial effect on source weight or proportion correct until the echo was attenuated by 18-30 dB. At echo delays < or =4 ms, source weights and proportions correct remained high regardless of echo attenuation. The proportions of responses predicted from the weights were lower at echo delays > or =16 ms. Results were discussed in terms of backward recognition masking and binaural sluggishness and compared to measurements of echo disturbance. 相似文献