Challenges of laser-cooling molecular ions

Electronic spectroscopy of a cold SiO+ sample: Implications for optical pumping

“…As mentioned, the well spectrally separated P, R-branch structure of the B 2  + -X 2  + (0,0) band facilitates rotational cooling via broadband laser source modified with a simple spectral mask 15 . This rotational cooling scheme relies on repeated removal of rotational quanta via the P-branch of the B 2  + -X 2 Σ + (0,0) transition, while at the same time avoiding having molecules fluoresce to levels other than the , and FCFs are also given in Table 1.…”

“…This technique was first demonstrated by vibrationally cooling Cs2 14 . It was also proposed that for a transition with diagonal FCFs, applying a sharp cutoff at the right frequency such that P-branch transitions are predominately driven will cause rotational cooling [15][16][17] . Rotational cooling by this method was first demonstrated 9 with trapped AlH + .…”

“…For the simplest implementation of broadband laser rotational cooling, near diagonal FCFs are important so that vibrational excitation is not likely to occur during the cooling process, although in some cases it should also be possible to re-pump vibrational excitations 15,18 [19][20][21][22][23] . In addition to electronic spectroscopy 19,20,22,24 , magnetic dipole allowed transitions between fine structure components of numerous rotational levels in the X 2  + (v=0) state were also measured by Rosner's group using a separated field optical pump/probes rf-double resonance technique 21 .…”

Prospects for Doppler cooling of three-electronic-level molecules

“…The approach of repumping by femtosecond laser pulse shaping increases the list of coolable molecules to include two-electronic-state hydrides [18], and certain molecules with a third intermediate state. For example, a low-lying A state also occurs in molecular ions: CO + [28], GeO + [29], SnO + [30,31], and PbO + [31]; and for neutral molecules: BO [32], AlO [33], GaO [34], InO [35], TlO, and N + 2 [36,37].…”

“…(The FCF matrix describes the coupling of different vibrational levels in an electronic transition, with diagonal FCFs allowing many absorption-emission cycles without vibrational repumping.) A number of two-electronic-state molecules have been proposed [13,14,18]; however, many potential candidates have an excited state whose lowest level lies above the dissociation limit of the ground state [18]. Molecules having this characteristic suffer from predissociation [19], in which coupling between the excited state and an unbound level of the ground state leads to dissociation on time scales which can be problematic for Doppler cooling [18].…”