High amplitude short time excitation

Cunningham, C. Glish, G.L. High amplitude short time excitation a method to form and detect low mass product ions in a quadrupole ion trap mass spectrometer. J. Am. Soc. Mass Spectrom. 2006,17, 81-84. 

Documented effects The alkaloid protopine (fumarine) caused narcosis in amphibians and, in mammals, caused paralysis of sensory nerve endings and increased reflex excitability. The alkaloid slightly increased the effects of analeptics and induced catalepsy (Chen-Gu 1957 Cheney 1963). In acute experiments with animals under narcosis, reduced heart rate and increased heartbeat amplitude occurred and, for a short time, decreased blood pressure was observed. Protopine has antiarrhythmic action with better effects than novocainamide and quinidine (Sadritdinov and Kurmukov 1980). In a screen to determine effects on platelet aggregation, extracts of this species showed complete inhibition of aggregation. This result was found to be caused by protopine (Sener 1994). Extracts of the dried plant displayed high rates of inhibition against the enzymes acetylcholinesterase and butyrylcholinesterase, which are associated with Alzheimer s disease (Orhan et al. 2004). 

In Figure 7.4a, we have displayed the probability of population transfer to vibrational levels in the Cs2 0 (65 + 6P3/2) excited state. During the pulse, many levels are populated, including highly nonresonant levels, due to time-energy uncertainty at short times. Rabi oscillations are visible diuing the pulse. Pulse transients are also observed, due to interferences between amplitudes of population coherently transferred to one level at different times they are discussed in Section 7.3.6. 

Fig. 5. Pulsed-nozzle FT microwave measurements. A molecule-radiation interaction occurs when the gas pulse is between mirrors forming a Fabry-Perot cavity. If the transient molecule has a rotational transition of frequency vm falling within the narrow band of frequencies carried into the cavity by a short pulse (ca. 1 (is) of monochromatic radiation of frequency v, rotational excitation leads to a macroscopic electric polarization of the gas. This electric polarization decays only slowly (half-life T2 = 100 (is) compared with the relatively intense exciting pulse (half-life in the cavity t 0.1 (is). If detection is delayed until ca. 2 (is after the polarization, the exciting pulse has diminished in intensity by a factor of ca. 106 but the spontaneous coherent emission from the polarized gas is just beginning. This weak emission can then be detected in the absence of background radiation with high sensitivity. For technical reasons, the molecular emission at vm is mixed with some of the exciting radiation v and detected as a signal proportional to the amplitude of the oscillating electric vector at the beat frequency v - r , as a function of time, as in NMR spectroscopy Fourier transformation leads to the frequency spectrum [reproduced with permission from (31), p. 5631.

An additional consequence of the widened spectral windows at high spectrometer field Is the requirement for enhanced pulse power. In order to assure uniform excitation across the full spectral width the rf pulses have to be sufficiently short. This Is particularly critical to achieve with hlgh-Q multlnuclear probes, and a fully satisfactory solution to this problem has at this time not been fomd. In Figure 5 the transmitter rf amplitude distribution for a... 

More recently, we have improved the time-resolution of the system substantially. The present instrument is capable of recording high time- and frequency-resolution spectra of transients having decay times from the nanosecond to the millisecond regime. The minimum time delay between the initiation of the transient and the first spectral observation can be arbitrarily short. (Typically, the first spectrum is recorded just before the transient in order to provide a background observation. A maximum of 128 successive time-delayed spectra of a single transient can be recorded the minimum time delay between each of these is 10 ns. All operational parameters (resolution, sensitivity, etc.) of the commercial Fourier transform spectrometer with which the system is used, are unchanged by time-resolved operation. Variability in die baseline due to amplitude instabilities in the excitation source (usually a pulsed laser) are taken into account, and appropriate corrections are made. 

Fig. 1.9 Absorbance changes can be measured with high time resolution by exciting a sample with a short pump pulse from a laser and measuring the transmission of a probe pulse that passes through the sample after an adjustable delay. The pump beam usually is interrupted periodically by a chopper, and the difference between the amplitudes of the transmitted probe beam with and without the pump light is averaged over a large number of pulses. The apparatus often includes additional lasers or optical devices for generating probe beams with various wavelengths or polarizations...

The spin-lattice relaxation, with a characteristic time Ti, is responsible for maintaining the population difference between levels, N and N+. The spin-spin relaxation time T2 reflects the lifetime of the excited state and its effect on the line width. If the electron-spin relaxation rate is too rapid, the lifetime of the excited state is short and the EPR spectrum becomes broadened. At high temperatures the spectrum may become too broad for detection, hence the use of cryogenic temperatures for some transition ions. However, if the spin-lattice relaxation is too slow, the population difference N - N+ cannot be maintained, and the amplitude of the signal is attenuated, a situation known as microwave power saturation. Electron-spin relaxation times may be estimated by measuring the amplitude of the signal as a function of applied microwave power. 

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