Two-Frequency Microwave Quenching of Highly Excited Hydrogen Atoms

Abstract

We present the first experimental results of quenching (ionization) of H(«o-43-49) atoms by a linearly polarized microwave electric field consisting of two frequencies (7.58 and 11.89 GHz). Adding a second frequency increases the quenching probability over that for only one frequency, especially where there is local stability at scaled frequencies near certain rational fractions, e.g., nfico -z and j. Some, but not all, two-frequency quench curves display a rich structure that appears to be related to that observed in some single-frequency quench curves.PACS numbers: 32.80.Rm, 42.50.Tj This Letter addresses a basic problem-how a bound quantal system responds to driving forces strong enough to rip it apart. Laser multiphoton ionization of atoms and dissociation of molecules are two prominent examples. We report here the first results 1 for ionization of hydrogen atoms H(A*O) with principal quantum number no^l by microwave electric fields consisting of two frequencies. Both fields may be strong. Lacking a theory directly relevant to these first "two-frequency" (2f) ionization experiments, we focus on data that we can interpret in the light of previous "single-frequency" (If) experiments 2 " 8 and theory. 9 " 22 Though this limits our present understanding to cases where one driving field is strong and the other significantly weaker, we expect it to stimulate interest in theory that is challenged by driving forces that rival the Coulomb binding force and by the need to treat final continuum states.Quantal "Floquet" theory 23 is useful for one periodic driving force with constant amplitude. Recently developed for finite -level systems, "multimode Floquet" theory 23 treats (rationally related) polychromatic driving forces, but there is some theoretical controversy 24 about the (chaotic?) response of even simple model systems. It appears that theory is not well advanced when, e.g., a continuum is involved, the driving frequencies are not rationally related, or the driving amplitudes vary in time.Let us recall some previous If ionization results. In atomic units the binding energy of level no is l/2«(?. A scaled frequency 9 ca/no~3 =n$co is the microwave frequency measured in units of the classical Kepler frequency (approximately the average of the no^>\ to no±l transition frequencies). This Letter reports data for no values between 43 and 49 and two fixed frequencies, a>i/2/r=7.5816(6) GHz and <»2/2;r = l 1.8893(6) GHz. Taken together these mean n §a> < 0.25, or more than four photons for spanning the no to no ± 1 interval, and a much larger number (l/ln^/co^no/Kn^co) > 100 for reaching the free-atom continuum. A scaled field 9 F/nQ~4 =noF is the microwave electric field amplitude measured in units of the initial Coulomb binding field. Let noF(X%) be that scaled field producing an ionization probability P[ on =*X%.Previous 9.92-GHz If ionization data 5 " 8 for « 0 == 32-90 covered the range n$co =0.05-1.1. Curves of noF(X%) vs nfico exhibited alternating local maxima (local stability) and minima (local instability)...

Dynamical localization in the microwave interaction of Rydberg atoms: The influence of noise

“…In contrast to the experiments of Bayfield and Sokol [19,20], Bayfield et al [23], Galvez et al [21], and Moorman et al [22] who work with fast beams of highly excited H atoms, our setup uses a thermal beam of rubidium atoms which can be laser excited to nP Rydberg states ranging from n =40 to ri 135. The laser radia- tion was produced by frequency doubling the light of a rhodamine 6G laser.…”

“…While in existing experiments on the microwave ionization of hydrogen Rydberg atoms [19][20][21][22][23] at least one of the three parameters is hard to change (e.g., the interaction time), our experimental setup is "universal" in the sense that all three control parameters can easily be varied. Out of the three parameters, the variable interaction time is of particular interest.…”

Experimental Evidence of Dynamical Localization and Delocalization in a Quasiperiodic Driven System

“…Working with a two-frequency quantum rotator thus allows one to go from the 1-d to the 2-d case by choosing irreducible frequency ratios corresponding to larger and larger q [7]. Experimental results on the twofrequency microwave ionization of Rydberg atoms have indirectly shown the importance of rational or irrational values in quantum transport properties [8].…”

Spontaneous and induced emission of a Rydberg atom in a cavity

“…Наиболее тщательные эксперименты по проверке применимости клас-сической динамики к процессу многофотонной ионизации атома водоро-да были выполнены в последнее время группой Коча [48,158]. Было показано, что в широком диапазоне параметров пороги ионизации хоро-шо согласуются с классическими вычислениями в одно-и двумерных системах [49,50].…”