Laser tunnel ionization from multiple orbitals in HCl.

Tunneling, one of the most striking manifestations of quantum mechanics, influences the electronic structure of many molecules and solids and is responsible for radioactive decay. Much of the interaction of intense light pulses with matter commences with electrons tunneling from atoms or molecules to the continuum. Until recently, this starting point was assumed to be the highest occupied orbital of a given system. We have now observed tunneling from a lower-lying state in hydrogen chloride (HCl). Analyzing two independent experimental observables allowed us to isolate (via fragment ions), identify (via molecular frame photoelectron angular distributions), and, with the help of ab initio simulations, quantify the contribution of lower-lying orbitals to the total and angle-dependent tunneling current of the molecule. Our results bolster the emerging tenet that the coherent interaction between different orbitals--which can amplify the impact of lower orbitals--must be considered in tunneling processes.