Context : Radio recombination lines ( RRLs ) at frequencies \nu < 250 MHz trace the cold , diffuse phase of the interstellar medium .
Yet , RRLs have been largely unexplored outside of our Galaxy .
Next generation low frequency interferometers , such as LOFAR , MWA and the future SKA , with unprecedented sensitivity , resolution , and large fractional bandwidths , are enabling the exploration of the extragalactic RRL universe .

Aims : We describe methods used to ( 1 ) process LOFAR high band antenna ( HBA ) observations for RRL analysis , and ( 2 ) search spectra for the presence of RRLs blindly in redshift space .

Methods : We observed the radio quasar 3C 190 ( z \approx 1.2 ) with the LOFAR HBA .
In reducing this data for spectroscopic analysis , we have placed special emphasis on bandpass calibration .
We devised cross-correlation techniques that utilize the unique frequency spacing between RRLs to significantly identify the presence of RRLs in a low frequency spectrum .
We demonstrate the utility of this method by applying it to existing low-frequency spectra of Cassiopeia A and M 82 , and to the new observations of 3C 190 .

Results : RRLs have been detected in the foreground of 3C 190 at z = 1.12355 ( assuming a carbon origin ) , owing to the first detection of RRLs outside of the local universe ( first reported in Emig et al .
2019 ) .
Towards the Galactic supernova remnant Cassiopeia A , we uncover three new detections : ( 1 ) stimulated C \epsilon -transitions ( \Delta \mathsf { n } = 5 ) for the first time at low radio frequencies , ( 2 ) H \alpha transitions at 64 MHz with a FWHM of 3.1 km s ^ { -1 } the most narrow and one of the lowest frequency detections of hydrogen to date , and ( 3 ) C \alpha at v _ { LSR } \approx 0 km s ^ { -1 } in the frequency range 55–78 MHz for the first time .
Additionally we recover C \alpha , C \beta , C \gamma , and C \delta from the -47 km s ^ { -1 } and -38 km s ^ { -1 } components .
In the nearby starburst galaxy , M 82 , we do not find a significant feature .
With previously used techniques , we reproduce the previously reported line properties .

Conclusions : RRLs have been blindly searched and successfully identified in Galactic ( to high order transitions ) and extragalactic ( to high redshift ) observations with our spectral searching method .
Our current searches for RRLs in LOFAR observations are limited to narrow ( < 100 km s ^ { -1 } ) features , owing to the relatively small number of channels available for continuum estimation .
Future strategies making use of a wider band ( covering multiple LOFAR subbands ) or designs with larger contiguous frequency chunks would aid calibration to deeper sensitivities and broader features .