We observe "trilobite-like" states of ultracold 85 Rb2 molecules, in which a ground-state atom is bound by the electronic wavefunction of its Rydberg-atom partner. We populate these states through the ultraviolet excitation of weakly-bound molecules, and access a regime of trilobite-like states at low principal quantum numbers and with vibrational turning points around 35 Bohr radii. This demonstrates that, unlike previous studies that used free-to-bound transitions, trilobite-like states can also be excited through bound-to-bound transitions. This approach provides high excitation probabilities without requiring high-density samples, and affords the ability to control the excitation radius by selection of the initial-state vibrational level.PACS numbers: 33.80. Rv, 33.20.Lg, 31.10.+z A class of long-range Rydberg molecules, sometimes called "trilobite molecules," occurs when a ground-state atom is embedded within the electronic wavefunction of a Rydberg atom [1]. The bond between the Rydberg atom and the ground-state atom originates from the attractive interaction between the Rydberg electron and the ground-state atom [1]. This bond has been described as a new type of chemical bond, distinct from the wellknown covalent, ionic, and van der Waals bonds [2]. The name "trilobite molecule" was coined because in certain states, the perturbed Rydberg-electron wavefunction resembles a trilobite fossil [1]. Trilobite states are characterized by large, and degenerate, values of orbital angular momentum (l ≥ 3) and large permanent electric dipole moments (EDMs ∼ 1 kDebye). Although pure trilobite states have yet to be observed, trilobite-like states, bound by the same novel chemical bond but characterized primarily by lower values of l and smaller EDMs, have been observed at ultracold temperature in photoassociation to bound vibrational levels [3][4][5][6][7][8], and at high temperatures in the form of satellite structures in the wings of atomic transitions [9,10].We adopt the name "trilobite-like" molecules for the low-l states to distinguish them from other types of longrange Rydberg molecules, such as macrodimers [11][12][13] or heavy Rydberg atoms [14,15].The bond length of a trilobite-like molecule can vary greatly depending on the principal quantum number, due to the n 2 dependence of the radius of the outermost lobe of the Rydberg wavefunction. For instance, the vibrational outer turning points of trilobite-like molecules have ranged from below 150 Bohr radii (a 0 ) in Refs. [9, 10] to above 10 3 a 0 in Refs. [3][4][5][6][7][8].Here we report the observation of trilobite-like states of 85 Rb 2 by a new method. We start by producing ultracold Rb 2 molecules in a high vibrational level of the metastable a 3 Σ + u state via photoassociation of atoms in a magneto-optical trap (MOT), then excite them directly to trilobite-like states, detecting them via their autoionization into Rb + 2 molecular ions. We observe these states in some of the lowest principal quantum numbers for which they can exist, n = 7 and 9-12, and...

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We report the production of ultracold RbCs molecules in the rovibronic ground state, i.e., X 'E +(i; = 0, 7 = 0), by short-range photoassociation to the 23n0 state followed by spontaneous emission. We use narrow-band depletion spectroscopy to probe the distribution of rotational levels formed in the X lL +(u = 0) state. We conclude, based on selection rules, that the primary decay route to X 'E +(i; = 0) is a two-step cascade decay that leads to as much as 33% branching into the 7 = 0 rotational level. The experimental simplicity of our scheme opens up the possibility of easier access to the study and manipulation of ultracold heteronuclear molecules in the rovibronic ground state.Tremendous efforts have been devoted to the produc tion of ultracold samples of heteronuclear molecules over recent years. Promising applications range from quantum computation [1] to ultracold chemistry [2], quantum dipolar physics [3,4], and tests of fundamental physics [3,5]. However, access to dense samples of ground-state ultracold polar molecules has so far been limited to a few outstanding ex periments [6,7], which utilized magnetoassociation followed by transfer to the ground state by stimulated Raman adiabatic passage [8]. A less experimentally complex path toward a sample of ultracold molecules is short-range photoassociation (PA) [9]. Recent discoveries of short-range PA transitions in RbCs [10], NaCs [11], KRb [12], and Rb2 [13,14] proved the general applicability of the process to alkali-metal dimers. Short-range PA is a convenient means to produce ultracold molecules due to its simplicity and possible continuous operation, although PA leads to an unavoidable distribution of molecules over many vibrational, rotational, and hyperfine levels. In fact, to date, short-range PA has predominantly produced molecules in rotationally excited states: ~2% of molecules were observed in J = 0 for LiCs [9] and no 7 = 0 molecules were observed for NaCs [11], However, it has been argued that simple measures may allow removal of excited states following PA [15,16]. Also, it is possible to increase the yield of molecules in the ground state by using vibrational cooling [17] and rotational cooling [18].In this paper, we perform high-resolution depletion spec troscopy [9,19] to measure the distribution of rotational levels in RbCs molecules produced via short-range PA to the 2 3n0 electronic state. We confirm that a large fraction of these X(v = 0) molecules, up to 33%, are in their rovibronic ground states, i.e., X(v = 0,7 = 0). We also show that the formation pathway to X(v = 0) is a two-photon-cascade decay as shown in Fig. 1 (a), as opposed to a direct one-photon decay as was previously supposed [16].Most of our experimental setup has been previously described [16]. 85Rb and Cs atoms are laser cooled and trapped in a dual-species forced dark spontaneous force optical trap (dark SPOT) [21] loaded by alkali-metal dispensers. The overlap of the two atom clouds is optimized 'Present address; PACS number(s): 37.10.M n, 3 3 .1 5 .-e , 3 3 .8 0 ....

Promising pathways for photoassociative formation of ultracold heteronuclear alkali metal dimers in their lowest rovibronic levels (denoted X(0,0)) are examined using high-quality ab initio calculations of potential energy curves currently available. A promising pathway for KRb, involving the resonant coupling of the 2(1)Pi and 1(1)Pi states just below the lowest excited asymptote (K(4s) + Rb(5p(1/2))), is found to occur also for RbCs and less promisingly for KCs also. The resonant coupling of the 3(1)Sigma(+) and 1(1)Pi states, also just below the lowest excited asymptote, is found to be promising for LiNa, LiK, and LiRb and less promising for LiCs and KCs. Direct photoassociation to the 1(1)Pi state near dissociation appears promising in the final dimers NaK, NaRb, and NaCs, although detuning more than 100 cm(-1) below the lowest excited asymptote may be required.

We report an upper bound to the ionization energy of 85 Rb2 of 31 348.0(6) cm −1 , which also provides a lower bound to the dissociation energy D0 of 85 Rb + 2 of 6 307.5(6) cm −1. These bounds were measured by the onset of autoionization of excited states of 85 Rb2 below the 5s+7p atomic limit. We form 85 Rb2 molecules via photoassociation of ultracold 85 Rb atoms, and subsequently excite the molecules by single-photon ultraviolet transitions to states above the ionization threshold.

We report continuous direct photoassociative formation of ultracold KRb molecules in the lowest vibrational levels (v ′′ = 0 − 10) of the electronic ground state (X 1 Σ + ), starting from 39 K and 85 Rb atoms in a magneto-optical trap. The process exploits a newfound resonant coupling between the 2(1), v ′ = 165 and 4(1), v ′ = 61 levels, which exhibit an almost equal admixture of the uncoupled eigenstates. The production rate of the X 1 Σ + (v ′′ =0) level is estimated to be 5 × 10 3 molecules/sec.

Studies of the proteome would benefit greatly from methods to directly sequence and digitally quantify proteins and detect posttranslational modifications with single-molecule sensitivity. Here, we demonstrate single-molecule protein sequencing using a dynamic approach in which single peptides are probed in real time by a mixture of dye-labeled N-terminal amino acid recognizers and simultaneously cleaved by aminopeptidases. We annotate amino acids and identify the peptide sequence by measuring fluorescence intensity, lifetime, and binding kinetics on an integrated semiconductor chip. Our results demonstrate the kinetic principles that allow recognizers to identify multiple amino acids in an information-rich manner that enables discrimination of single amino acid substitutions and posttranslational modifications. With further development, we anticipate that this approach will offer a sensitive, scalable, and accessible platform for single-molecule proteomic studies and applications.

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